Peter told you how to quickly set desktop effects in KDE with the press of a button if you're using KWin. A similar trick is available for GNOME's Metacity.
First, you'll need to be using Metacity, and not Compiz. Yes, Metacity has compositing.
Metacity's compositor can be turned on in GConf, but for the enlightened, it's an Alt+F2 and a command away:
And to disable again:
It's not quite the magical three-key shortcut KWin has, but it sure beats going into GConf to tick a checkbox. It's the perfect lightweight replacement for Compiz if you don't need all of the bells and whistles but still want to use some composited applications like GNOME Do and Screenlets.