Rogue Amoeba’s AirFoil started out as a way to stream any non-iTunes audio to your AirPort Express mini-router, back when AirPlay was still called AirTunes. Then it was expanded with a free iOS app which would let you stream music from AirFoil on the Mac to AirFoil on your iPod or iPhone, handy for hooking up to a stereo.
Now we have AirFoil Speakers Touch 3 for iOS, and it adds in proper AirPlay support, letting you send music from pretty much any iDevice you own.