xGestures is similar to Cocoa Gestures, only it works for all applications, not just Cocoa ones. Its settings are accessed through a System Preferences pane (much easier than within each application, as in Cocoa Gestures). It supports multiple applications. Application specific gestures override global gestures (as far as I can tell), which makes exceptions easy to deal with. I use it in conjunction with Cocoa Gestures, assigning middle click to Cocoa Gestures, while leaving right click available to xGestures.
I have been using this for about six months, and it is possible to almost completely eliminate the keyboard as necessary for input. It saves a lot on mouse mileage, too. Those tiny buttons are no longer a pain to click.
I couldn't find it listed anywhere on the site and thought it would be a good addition. It is definitely the most useful input modifier I have ever used. I have a laptop, and this lets me use my computer from a distance with my bluetooth mouse, keeping that hot thing off my lap and only leaning forwards to type (all my shortcut keys are taken care of with mouse gestures).
[robg adds: Cocoa Gestures has been replaced with the shareware CocoaSuite, which goes beyond gestures and adds menu shortcut assignments, mnemonics, and virtual scroll wheel support. I tested xGestures, and it definitely works in all applications -- note that it installs a kernel extension to do so, but it's simple to disable and/or remove if you have troubles (I didn't experience any issues with it).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060622142225417