We recently covered a utility called MagicPrefs that adds a hatful of new and configurable multi-touch gestures to your Magic Mouse. Well, there's another free utility called BetterTouchTool that works with both trackpads and Magic Mice, providing a huge number of gestures and infinite possibilities for configuration. BetterTouchTool is a fork of the MultiClutch project, built for Snow Leopard and expanding the feature set greatly. It's the work of Andreas Hegenberg (also the author of SecondBar), and it's come a long way since the first alpha version I tested a while back.
Aside from improved stability, the interface is one of the elements that's come the farthest. It's now quite easy to start adding gestures such as 'Three Finger Swipe Down' and 'Four Finger Click,' and assign them to predefined actions like 'Show Desktop' or 'Open Finder.' You can also assign any shortcut key you like, or assign a gesture to launch any application ... meaning you can make just about anything happen. BetterTouchTool also has actions available that control window sizing and movement, ala SizeUp, but with gestures.
Groups of gestures can be assigned to specific applications, or made global. The latest version includes presets (which you can switch between), import, and export. Separate gestures are available for the trackpad and the Magic Mouse, and the new "Live View" feature makes it very easy to gain precise control over sensitivity settings and to test out gestures. BetterTouchTool is free, and is likely to remain that way, according to the author. It's still got the occasional glitch, but development is rapid and it's improving and expanding almost daily. If you've got a MacBook or MacBook Pro with a multi-touch trackpad, a Magic Mouse, or both, give it a shot!