Mantis Bug Tracker (MantisBT) is a Web-based issue tracking system that is both simple to use and highly customisable. A plugin system allows users to easily create and add new features. MantisBT is over 10 years old and is actively developed.
Trac is a minimalistic but highly useful issue tracker and
software project environment based around an integrated Wiki
engine. Features include an interface to Subversion (source
revision control), a bug/issue tracking database, and
convenient report facilities.
Whups is a Horde ticket-tracking application. It
is very flexible in design, and can be used for
help-desk requests, tracking software development,
and anything else that needs to track a set of
requests and their status.