An AirPort Express will let you listen to iTunes over your home stereo speakers. Airfoil is an application that takes that simple functionality and turns it up to 11. The iTunes-AirPort Express combo is perfect for listening to iTunes on your nice stereo, but what if you want to hear music from a browser-based app like Pandora? Or Sirius Radio through an app like StarPlayr? Or work with audio from your Elgato EyeTV? Airfoil lets you send audio from any application (even your system audio) to your AirPort Express, so you can use your stereo for anything from RealPlayer and QuickTime to browser-based audio like Pandora's. When you Option-click the source menu, you can choose from all background processes, including virtualization programs like Parallels or VMWare. If you have Airfoil Speakers Touch, you can even send audio to your iPhone or other iOS device.
Airfoil also lets you send synced audio to multiple AirPort Express units all over your house, as well as to Apple TVs and even other local computers; Mac or PC, all you have to do is install the free Airfoil Speakers mini-app. Airfoil Speakers has gotten even better in version 4: you could already launch audio directly from it (without launching Airfoil first) and use basic playback controls, but now you can control Airfoil remotely from AirFoil Speakers with sources like iTunes and QuickTime.
Airfoil comes with a built-in video player, too, that lets you play video on your computer with synced audio, so you're no longer stuck with using headphones or your built-in computer speakers to watch movies. Airfoil Video Player also lets you watch video from Netflix or Hulu while keeping the audio in sync with your speakers.