Bugzilla
Bugzilla is a general-purpose computer bug-tracking tool developed by, and used by, the Mozilla team.
Being web based and open source has made it the bug tracking tool of choice for many open source projects, and quite a few commercial ones.
Bugzilla relies on an installed HTTP server (such as Apache) and a database (usually MySQL) to perform its work. Bugs can be submitted by anybody, and will be assigned to a particular developer. Various status updates for each bug are allowed, together with user notes and bug examples.
The notion of a bug in bugzilla is very general; for instance, it can be used to track feature requests as well.
