Bugzilla is a very popular open source bug tracking system written in Perl. It is both immensely powerful, scalable, customizable, and immensely hard to use for the average non-technical user. For this reason it is particularly popular within hardcore "hacker" communities, but is also the antithesis of "Web 2.0" design.
I used it for several years for tracking bug reports and feature requests at: http://bugs.wincent.com/
- Official site: http://bugzilla.org/