Utilize extra mouse buttons in Mac OS X

If you just want the side buttons to do back/forward in ANY app/environment, this app is the most basic, streamlined one I've found for this purpose on Mac OS X, and completely free (donate!). http://sensible-side-buttons.archagon.net/

There are a few apps that can do this task. I use SteerMouse [$20] which can handle mice with up to 15 buttons.

The latest v5 can now use 'chords' [multiple buttons to generate a separate action] & modifier keys - that gives a potential 24 actions for every button, different for every app if you wish... if you can ever think of that many ;)

USB Overdrive is another I can think of, but I've never used that one, so can't comment.