SideTrack turns your trackpad into a multi-button mouse with minor tweaks to your Mac OS X System Preferences menu via SideTrack. Full horizontal and vertical scrolling is enabled on user defined edges of your trackpad.
You can download an evaluation version of SideTrack for free, which opens a reminder window every odd moment to bug you to register. Registering SideTrack costs USD$15.
Another product from Raging Menace worthy of mention is MenuMeters which integrates system monitoring into your Mac OS X menubar. Details of memory usage, hard drive activity lights and bandwidth usage through your active IP connection. The only thing missing is CPU temperature monitoring. To fully enjoy MenuMeters, a 14″/15″ iBook or PowerBook is recommended.