This.
It's just excellent.
You can configure it for every single application. Using Firefox I have history back/forward buttons, next tab/last tab functionality (middle mouse wheel to the right/left), open link in new tab/close tab (push middle wheel), a search Google button, a thumb wheel which opens a pop up where I can switch to any currently running application and a scrolling wheel which, dependent on its spinning speed, continues scrolling or does scroll steps.
Hell, I can use the computer without even moving the mouse!