SoundSource Provides Fast Audio Input and Output Switching - Audio - Lifehacker:
"SoundSource is a free download, Mac OS X 10.5 and above only.
Update: As many of you have pointed out in the comments, some of this functionally (specifically, the ability to switch input and output sources) is already available in the menu bar if you option click the volume icon (Snow Leopard only). If that's all you need, then do that instead—no need for extra software. However, the added ability to adjust volume levels and access, say, MIDI setup is a nice addition if you're doing something a bit more involved (or if you're still on Leopard)."