Couple of quick ones today:
First, Macworld senior editor Dan Frakes recently reported that he'd inadvertently stumbled on an undocumented keyboard shortcut for deleting messages in Mail: Control-H. Hitting that key-combo does the same thing as the Delete key, sending messages to the Trash folder.
That might seem odd at first glance, but isn't really odd at all: Control-H is the Unix keybinding for the Backspace or Delete key. As a result, you can use Control-H to delete text in apps that use the Cocoa text input framework. Its ability to delete messages in Mail is an interesting side-benefit.
The second tip (from an anonymous Hints reader) is for those of you who use two calendar applications. If you do and if, say, you're switching from Outlook to iCal, you can drag an event from Outlook into iCal's window, and it will be duplicated in iCal. If you drag the event onto the iCal icon in the Dock, you'll get a dialog asking which calendar you want to use (assuming you have more than one), and the event will be added to it.