xGestures is an application for Mac OS X that enables to control any Mac application with mouse gestures.
There are others also (like cocoa gestures) but this one seems to be the best. It even recognizes ‘rocking’ gestures (like I only knew them from Opera), i.e. clicking one mouse button and then another one before releasing the first one. Interaction-wise, i think this is the fastest and most effortless way to e.g. go back anf forward in a web browser.
It can be freely configured for any app so you can use mouse gestures to close windows, switch applications, control music – anything really.
It’s $5,– once the trial expires. I’ll definitively pay.