Back in the 10.4.3 minor update, Apple silently added a pretty interesting feature to OS X. On the login window, below the large silver Apple logo and the words Mac OS X, you normally see the name of your Mac. While interesting, this isn’t necessarily all that useful. After all, you’re logging into the machine, sitting right in front of it, so you probably know which Mac it is—unless you’re one of those surrounded by six Macs all sharing one keyboard and monitor via a KVM switch.
When 10.4.3 came out, that info line was suddenly not quite so useless. You can now click your mouse on your machine’s name, and the OS X version number will appear. Click again, you’ll see the OS X build number. Keep clicking, and you’ll see your OS X serial number, the Mac’s IP address on the network, the status of any network accounts from OS X Server, and finally, the current date and time. Here’s a brief movie of what the various messages look like:
This can be very useful for troubleshooting a friend’s system, or for tech support folks trying to help others (which is probably why Apple added the info in the first place), or just plain useful—I personally find the date to be much more useful to me than I do the name of the machine. Unfortunately, the changes you make won’t stick around. Change the display to date, and the next time you login, it will be back to the machine’s name. But with a tiny bit of work in Terminal (in /Applications -> Utilities), you can make a permanent change.