Growl is an open, global, notification system for Mac OS X. What this means is that any app can plug into it in order to send status notifications that pop up on your screen.

Growl is an unobtrusive way to avoid the Pavlovian response to seeing the unread count in Mail go up in the Dock. Instead, you'll see a brief (configurable) popup that gives you a snippet of the mail message as it's received. Which means, if you're like me, you won't necessarily be interrupting your work to switch back and forth to and from Mail.

There are a ton of applications that plug into Growl to deliver various notifications, although the only other one that I really use is CyberDuck. Still, very highly recommended since it's both free and extremely useful.