Red Hat Bugzilla is probably the most popular Bugzilla variant on the planet. One of the major benefits of Red Hat Bugzilla is the ability to work with Oracle, MySQL, and PostGreSQL databases serving as the back-end, instead of just MySQL. Dave Lawrence has worked very hard to keep Red Hat Bugzilla up-to-date, and many people prefer the snappier-looking page layout of Red Hat Bugzilla to the default Mozilla-standard formatting.
URL: http://bugzilla.redhat.com/bugzilla/