
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 10.6 (Snow Leopard) and either the Magic Mouse or multitouch capable MacBook or MacBook Pro to use.