NONTROPPO XGestures is the best $5 you can spend on software for OS X. Mouse gestures are an incredibly effective interface addition. I normally work in mixed keyboard / mouse mode but sometimes I'm far from the keyboard. Gestures allow full navigation and triggering of all your apps. Navigate in Finder, Forklift, Safari, FF using the same gesture for back, much faster than using their back buttons. Toggle side panels, run applescript.
The only issue is with Wacom tablets. The developer knows about this and will try to fix it once he geta a tablet to use. I stopped using my Wacom tablet, as I found disabling XGestures just to disruptive to my overall workflow!
(Version 1.7.3)