B.3. RT

RequestTracker's web site says "RT is an enterprise-grade ticketing system which enables a group of people to intelligently and efficiently manage tasks, issues, and requests submitted by a community of users," and that about sums it up. RT has a fairly polished web interface, and seems to have a pretty wide installed base. The interface is a bit visually complex, but that becomes less distracting as you get used to it. RT is licensed under the GNU GPL (for some reason their web site doesn't make this clear).

http://www.bestpractical.com/rt/