One great asset to any Macintosh computer is Growl. There really is no other application like it. It’s a very customizable alert/notification system which integrates with many applications. You can make it notify you when a download is complete, or let you know what song is currently playing on iTunes. It even tells you when your computer maintenance is complete, or all kinds of activity running on your chat client.
You can also customize the appearance of the notifications, be it from Bezel to Smoke, and more can be downloaded. It’s a very elegant addition to the world’s most elegant OS 😛