Sidetrack is a beta, open source replacement driver for the MacOS X trackpad drivers that adds a lot of cool power-user features to your trackpad:

* Vertical scrolling at left or right edge of pad.
* Horizontal scrolling at top or bottom edge of pad.
* Map hardware button to left or right click.
* Map trackpad taps to no action, left click, left click drag (with or without drag lock), or right click…

Link (Thanks, Joel!)