Mac only: Whether you're using Apple's new Magic Mouse or a multitouch-capable MacBook, you may be disappointed with the small number of gestures available. BetterTouchTool expands the stock multitouch features by adding gestures and letting you choose what they do.
BetterTouchTool adds tons of new gestures to your mouse such as tapping in the corner, tapping with multiple fingers, swiping in different directions, and, in the case of the Magic Mouse, even pinching and stretching to zoom in and out. Each of these available gestures can be set to correspond to either a keyboard shortcut or a predefined action, such as Exposé or Finder. The developer updates the app extremely often as well, so it's constantly improving.
BetterTouchTool is a free download, requires Mac OS X and either the Magic Mouse or multitouch capable MacBook or MacBook Pro to use. Thanks, PatrickHoulihan!