Easystroke is a gesture-recognition application for x11. it aims to be highly configurable while at the same time providing an intuitive user interface. it was designed with a tablet pc in mind, but it also works well with a mouse.
I've been using Easystroke for a while now, and I must conclude that it's a great little program! Only the tray icon is a bit useless, there'd better be an option in the configuration menu and the control center.