Is there any way to set a MacBook Pro to not sleep when you close the lid? The most common use case I have for this is when I'm playing music over AirTunes, but don't want an open laptop laying around.   Or when I'm syncing or downloading something.

This isn't the same as this question, which related to sleep vs. hibernation; I'm looking for a way to make it stay active with the lid closed.
You can use InsomniaX. It sits in your menu bar, and upon activating it, your laptop won't sleep if you close it.