Apple updates its iOS mobile operating system once a year. But why should the iPhone and iPad have all the fun? On Thursday Apple announced that it will release a new version of OS X—Mountain Lion—this summer, just a year after the release of OS X Lion.

Like Lion, Mountain Lion offers numerous feature additions that will be familiar to iOS users. This OS X release continues Apple’s philosophy of bringing iOS features “back to the Mac,” and includes iMessage, Reminders, Notes, Notification Center, Twitter integration, Game Center, and AirPlay Mirroring.

Mountain Lion offers new features such as (left to right) Notes, Reminders, Messages, and Notification Center.

As the first OS X release post-iCloud, there’s also much more thorough integration with Apple’s data-syncing service. Mountain Lion also brings options to limit which kinds of apps users can install. And although there are no actual mountain lions in China, OS X Mountain Lion does add a raft of features to speak to users in the country that’s Apple’s biggest growth opportunity.

iChat is dead—long live Messages. With Thursday’s announcement of Mac OS X Mountain Lion comes the news that iChat is being upgraded and renamed to Messages, with support for the iMessage chat system introduced with iOS 5.

If you can’t wait until Mountain Lion arrives this summer to finally use iMessage with your Mac, relax—you don’t have to. On Thursday, Apple also released a beta version of Messages for Lion users. (The final version will be available this summer when Mountain Lion ships.)

I’ve spent the past few days using Messages and Mountain Lion. Here’s a first look, keeping in mind that Mountain Lion won’t be released for months, so features are in flux and could change.

It uses iMessage

The actual wording is in flux, but Mountain Lion will warn you if you try to open an app from an unidentified developer.

Last year saw the arrival of the Apple-curated Mac App Store, a creation very much in the mold of the iOS App Store. And many people wondered: Would a locked-down version of Mac OS X, one incapable of running apps not approved by Apple, be far behind?

Apple certainly could have done something like that with Mountain Lion, the company’s planned update to Mac OS X that should arrive this summer. But it hasn’t—instead, the company has created a new way for developers to sign their work and a new set of options in the Security & Privacy preference pane. According to Apple, it’s all an attempt to improve Mac security. Here’s how the new Gatekeeper feature works.

Check before first launch

Since at least the days of version 10.5, OS X has had a feature called File Quarantine, which checks apps before they run for the very first time. You most often encounter this feature when you download an app and run it for the first time—a dialog box appears informing you that it’s a file downloaded from the Internet, and asking for you to confirm that you do indeed want to run it. (This feature only works with files downloading by certain apps, including web browsers and email clients. A file copied from a USB drive or from a network volume doesn’t get checked.)

For years, many Mac app developers have had to design their own ways to get your attention. The open-source project Growl helped save those developers from reinventing the wheel by creating a more general notification system supported by lots of apps. But with Mountain Lion, a true systemwide notification service will finally arrive when the Mac OS X update ships this summer.

Like its counterpart on iOS, this new service is called Notification Center. With Notification Center, Mac developers now have access to features much like those already found in iOS.

Alerts and notifications appear in the top right corner of the screen in a small bubble that looks an awful lot like a Growl notification. Notification bubbles remain there for five seconds, and then slide off screen to the right. Alerts, on the other hand, remain on-screen until you click on the Show or Close (or in the case of some alerts, Snooze) buttons.

In iOS 5, you see all your recent notifications by pulling down from the top of the screen to reveal Notification Center. In Mountain Lion, the Notification Center list is a narrow band that lives just to off the right side of your screen. You can reveal it either by clicking on the new Notification Center icon at the far right of the menu bar, or by swiping with two fingers starting at the far right edge of the trackpad. Either way, your Mac’s entire screen will slide to the left, revealing a list of what’s been trying to get your attention recently.

Two of the new iOS-flavored apps to move to the Mac with the release of Mountain Lion this summer are Notes and Reminders. Here’s a sneak peek at how they work.

Notes

As in its iOS incarnation, the Notes interface on the Mac is dominated by a yellow text-entry area that resembles a legal pad. There’s even a hint of torn paper at the top of the window, and yes, the app’s title bar offers a leather texture. The top right corner displays the date on which the note was last modified.

At the bottom of the text-entry area, there are just two buttons. A Trash icon deletes your note; the Share icon brings up one of Mountain Lion’s new Share Sheets, offering to attach your note to an email message or an iMessage.

Messages is now ready for Mac users to test drive. Shortly after announcing its planned Mountain Lion update for OS X on Thursday, Apple released a beta of the revamped version of iChat that will ship with the new operating system this summer.

You can download the Messages beta directly from Apple.

Messages draws its new moniker—and much of its functionality—from the identically named app in iOS. It will replace iChat in Mountain Lion, incorporating all the instant messaging features of the mobile app including the ability to communicate with folks using iMessage. iMessage conversations sync automatically between iOS devices and the Messages app on the Mac.

Our own Jason Snell spent several days using Messages already, and has a full hands-on.

Thursday’s surprise announcement of the next version of Mac OS X had developers across the Mac community perking up their ears, thanks in no small part to a new feature in Mountain Lion called Gatekeeper.

“My takeaway on Gatekeeper is it’s a lightweight introduction of the notion of registered developers outside the App Store,” Daniel Jalkut, proprietor of Red Sweater Software, explained to Macworld.

Gatekeeper relies on a technique called code-signing, in which software developers are provided with a cryptographic certificate from an authority—in this case, Apple—which they can then use to digitally “sign” their applications. It’s similar to the process that consumers encounter when they buy things via the Web, in which they’ve been trained to look for the padlock icon that indicates a secure transaction.

“Security based on digital signatures has been a long time coming, so it shouldn’t be much of a surprise to developers,” said Ecamm Network co-owner Glen Aspeslagh. “As the Mac gains in popularity, Apple’s approach will be a powerful and much needed weapon against malware.”

While our Windows-using compatriots have been plagued by malware of all shapes and flavors, Mac users have remained largely unscathed, although the debate continues to rage over whether that’s because of innate superiority in the Mac operating system or the Mac’s smaller market share presenting a less tempting target for writers of malicious software.

If our website comments and email inboxes are any indication, among the least-popular changes in Lion (OS X 10.7) were those performed on iCal and Address Book. From their unnecessarily skeuomorphic interfaces to their omission of features available in Snow Leopard, these two apps have, in many a user’s mind, earned the criticism they’ve received.

Which explains why many Mac users will welcome the updated versions of iCal and Address Book—now called Calendar and Contacts, respectively, for consistency across all of Apple’s computing devices—provided in Mountain Lion. While they don’t fix all issues present in their Lion counterparts, each offers a number of significant improvements. Here’s a quick look.

Contacts

While many iOS users will welcome Contacts based on the name change alone—especially if, like us, you often attempt to launch your address book in Spotlight by typing contacts—the biggest improvement to the app is the return of the three-column layout. Gone is the need to click a virtual bookmark to ever-so-slowly turn virtual pages; instead, you can choose from three display modes: card (single column); card and contacts (two columns); or card, contacts, and groups (three columns). To retain the app’s book-like design, the three-pane view narrows your contacts list, allowing the Groups sidebar to sit on the far left of the page.

Contacts additionally sports Mountain Lion’s new Share button at the bottom of each card; you can export that card (in .vcf format) as an email or iMessage attachment, or via AirDrop.

If you heard that iChat is dead, fear not. It’s alive as ever, but under a new name: Messages. Apple announced on Thursday that when Mountain Lion arrives in late summer, the re-christened, re-icon’ed Messages app will become the new stock instant messaging option. Messages retains all of iChat’s old functionality, but it adds in several new abilities: an option to exchange iMessages, a direct hook into FaceTime, and an updated chatting interface. Apple released a beta version of Messages for Mac users running Lion (at least version 10.7.3).

In our hands-on, we've shown you what you can expect when you download the beta, but now we can give you the rundown on what Messages gets right and what still needs some work before a final version arrives this summer.

Apple has plenty of time to make changes, so it’s entirely possible that Messages could look markedly different by the time Mountain Lion lands. And the current release really is beta software; you might encounter some crashes or occasional buggy behavior. But here’s our take on the more substantive features of the beta.

What works: The new look

The incorporation of iMessage is theoretically the big news with the new Messages app. But as significant as that addition is, and we’ll cover it below, perhaps the most immediately apparent change is the app’s redesign.

Though Mountain Lion snuck onto the scene just this morning—big cats are sly like that—I’ve already discovered a few delightful details about the apps in Apple’s upcoming OS update for the Mac, thanks to editorial director Jason Snell’s hands on with the software. Here are five app changes I can’t wait to see come to my Mac this summer.

Souped-up Safari searches

The Safari toolbar has been reworked in Mountain Lion.

Safari gets a few nice tweaks in Mountain Lion, but none so nice as its new-and-improved search feature, now built directly into the URL bar. Like a Transformer, it can alter itself to fit your needs, whether you’re searching the Web, your bookmarks and history, or text on the page itself. You can also enter in a URL the good old-fasioned way, or type in a piece of text and have Safari return the top-rated search result for that phrase.

Notifications will float on your screen as banners or pop-ups.

Sleek notifications

Growl is great for the apps it supports, but I personally can’t wait to see Notification Center pop up on my Mac. Like iOS, you can customize in System Preferences which apps alert you; whether they have banners, pop-up dialogs, sounds, or nothing at all; how many alerts per app you’d like to see; and whether you want those apps to also feature badge notifications on their Dock icon. Combine this with a new two-finger gesture (swipe from the right edge of the trackpad inward) and an optional keyboard shortcut, and alerts for things like Mac App Store updates, and organizational bliss could soon be yours.

Any time there’s a new operating system, there’s bound to be lots of questions about new features and capabilities. And when that operating system’s unveiling is as surprising as this week’s Mountain Lion announcement was, those questions take on a newfound urgency.

Not to worry: We’ve had a chance to spend some quality time with the next version of the Mac operating system, and we’re prepared to answer any questions you might have about its availability, new features, and enhancements to existing applications. And don’t worry if you’ve still got lingering questions—we have a few months before Mountain Lion arrives to sort everything out.

Availability

When will Mountain Lion be available?

Well, there’s a developer preview of Mountain Lion available now, so that software makers can update their apps to take advantage of OS X’s new features. As for end users like the rest of us, when it announced the developer preview via press release, Apple specifically said “late summer 2012.” That’s not a precise date, so Mountain Lion could leap into action any time before the Autumnal Equinox on September 22. For your historical reference, Lion arrived on July 20 last year, while Snow Leopard shipped on August 28, 2009—both those OS X updates were given initial release dates of “summer” as well.

How will I get my hands on Mountain Lion?

Though Safari didn’t receive as huge an overhaul in Mountain Lion as other OS X apps have, it still got some love from the folks in Cupertino. Refined search interfaces, sharing integration, and clear-cut password storage are just three of the things you have to look forward to come this summer—here are a few brief impressions of Safari’s new features.

A new look

Safari has a new unified search bar.

Those who favor the “one big search bar” approach to surfing the Web will be pleased to see that Safari’s toolbar has taken just that cue: The browser now sports a single lengthy field that can be used to type in a URL; pull up the top result in your selected search engine from a keyword; or search the Web, your bookmarks and history, or within the page itself. URLs themselves have taken on a slightly Chrome-esque look, automatically removing the “http://” at the beginning of the link and graying everything in the URL following the root domain.

To the left of this bar is Mountain Lion’s new Share button, which you’ll see popping up just about everywhere in the operating system when it launches later this year. Share in Safari currently only offers adding to your Reading List, adding a bookmark, emailing the page, iMessaging the link, or sending a tweet about it; Apple may expand this list at some point in the future, but there’s currently no way I could see to customize it.

Safari has ditched RSS support, and replaced the RSS button with a large one for Safari Reader.

When Apple released Lion (OS X 10.7), Mail received its biggest overhaul ever, gaining many new and noteworthy features. The changes to Mail in Mountain Lion are more subtle, though there are some nice improvements—along with one significant omission.

Inline find

Mail has always let you find text within the body of an email message, but in the current version of Mail, you use a separate Find dialog that highlights only a single result at a time; to find subsequent instances of your search string, you repeatedly click the Next button (or press Command+G). Mail in Mountain Lion gains a Safari-like inline find feature: Press Command+F (or choose the Find command in the Edit menu) and type your search string—in a field just above the body of the current message, rather than in a separate window—and the body of the message is dimmed, with every occurrence of your search string displayed. You can use the arrow buttons next to the search field, or their keyboard-shortcut equivalents, to cycle through occurrences.

Mail's new Find feature is reminiscent of Safari's.

You don't lose the capability to use Find & Replace, however. Whenever you do an inline find while composing a message, a Replace checkbox appears; check that box, and you can enter your replacement text, with options to replace just the current instance or all instances of the search string. The location of the find/replace fields within the current message window is more convenient than—and reduces the screen clutter due to—a separate Find window. It's also less confusing, as you'll never see a Replace option for an incoming message. On the other hand, you lose the Lion option to perform case-sensitive searches, but we suspect this won't be an issue for most users.

Notifications in rules and selective notifications

Mail's Notifications options