Mouse and hand gestures are a semi-rare feature we see pop up in games or applications from time to time. They are starting to become more popular, though, as people seek to break out of the traditional point-and-click way of interacting with PCs. More recently, Mozilla has delved into experiments using the stock gesture support that is found on the newest MacBooks, and now using the pre-beta version of Firefox 3.1, people can tinker with gestures in the browser.
Mozilla hasn't planned this for a final release of Firefox 3.1, but is certainly encouraging feedback at this point. The gestures take place on the trackpad as opposed to the screen, and so far include only the most basic functions: going forward or back, going to the top or bottom of a page, et cetera.
Interacting with computers via gestures is a very nifty idea. Unfortunately many gesture systems are more cumbersome than sticking to a mouse and keyboard, so there aren't many viable ones in place yet.