audioIOchanger

This functionality is out-of-the-box in Snow Leopard. Simply option-click the volume icon.

@Bytesmiths: Is it still available? The dev has moved to a new site not yet complete, but AIOc isn't among his future apps.

@Longboy, not sure what you mean, or perhaps you put the wrong person in your reply. Simply option-click the little speaker in the menu bar of any Mac since Snow Leopard, and you'll get a menu of sound sources to select from. I was searching for such a thing, and the discovered it came from Apple! So I suspect the developer has ceased working on it, since it now comes with every copy of Mac OS.