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.