AirTunes allows you to play your music from your computer to a stereo wirelessly. The one down-side with AirTunes is that it only works with iTunes. You can play your iTunes music on your stereo, but if you have music from other sources (Pandora, Last.fm, RealPlayer, Windows Media Player, QuickTime Player, etc), you can't play them on your stereo using AirTunes.

But Airfoil solves this problem. It's a piece of software (available for both the Mac and Windows machines), that allows you to stream any audio source to your AirTunes and then ultimately to your stereo. It even allows you to transmit to multiple AirTunes setups if you have them.