*Update - Apple's Snow Leopard partially negates the need for SoundSource. Option-click on the volume icon in the menu bar (turn on in System Preferences) gives you quick access to your inputs/outputs. SoundSource still has a bit of an advantage by auto switching when you plug in headphones.

Working in an office environment I spend a lot of time listening to music through my headphones. When the time comes that I need to show something requiring sound I switch to my external speakers. This was a bit of process before—control + space, S Y S, return, sound, output, device.

Needless to say that I'm really stoked to have found this free app. from Rogue Amoeba - SoundSource. It puts an icon with a drop down menu on your menu bar.  Sexy!