I just found this cool site that can take the serial number of your Apple hardware [computer, iPod, etc] and tell you more about the item -- the Apple stock number, which factory it was assembled in, the week it was made, and other little factoids.

Handy in a pinch: I was trying to diagnose why a friend's MacBook wouldn't boot when she put 4GB of RAM in it ... turns out that her model only supported up to 2GB, and she had mistakenly thought it could take 4GB.

Remember that you can find the serial number by clicking a couple times on the version number text underneath "Mac OS X" in the About This Mac dialog, or simply in the Hardware Overview pane of the System Profiler app.