Bugzilla is an
open source bug tracking system written in
Perl using the
mysql database system, which is also
open source.
Because it's oipen source it's extremely extensible. My company has made a number of modifications which
we hope to release back to the Mozilla project at some point.
Also a result of its open nature a number of integrations to other systems exist, including a rather nice perforce integration for meshing your revision control/SCM and bug tracking systems.
Bugzilla's human interface is rather nice. As a software developer you get e-mail saying a bug has been assigned to you. You fix the source code that the bug pertains to and then mark the bug fixed with a little comment saying what you did. That's all.