Best Practical's Request Tracker.
Fantastic ticketing system; we've been using it since version 1.0 and just recently merged five independent help ticketing systems into one RT server. It's open source, runs on modest hardware, is fast, expandable, easy to use, works with all browsers, etc etc etc.
Plus there's an O'Reilly book that talks about it -- how cool is that?!
http://www.bestpractical.com/rt/