Mac OS X only: MagicPrefs lets you create custom gestures (and override existing ones) to provide new multitouch functionality to your Apple input devices. It's like the great Multiclutch, which serves a similar purpose, but with added power and precision that gives you serious control over even the nuances of your gestures.
You have five fingers and MagicPrefs recognises that, letting you assign actions to taps and clicks based on the number of fingers present on the mouse. Additionally, it supports swiping, rotating, and pinching actions. You can make these actions do carry out tasks like zoom in and out of the screen, start Exposé or Dashboard, initiate Quicklook, and even run AppleScripts. If you find a gesture is too sensitive, or not sensitive enough, you can adjust that. You can even create presets for specific circumstances, so you can assign certain gestures for when you're working and others when you're gaming.
MagicPrefs gives you a lot room to fine-tune your customisations, and it's free to download.