Growl

Growl fixes a gaping hole in MacOS X. Back in the OS 9 days (and at least as far back as OS 7), the Mac had something called the Notification Manager. OS X is better than its predecessors in almost every way, but the way OS X allows applications running in the background to notify the user that something has happened sucks compared to Notification Manager.

Growl will save you from that. Growl is a free, open source notification giblet for MacOS X. Lots of cool software supports Growl. Apple ought to hand the Growl developers a big pile of money and incorporate Growl into the OS.

If you have a modern Mac and you aren't running Growl, then go get it. I'll wait here.