Issuezilla is another fork from Bugzilla, and seems nearly as popular as the Red Hat Bugzilla fork. Some Issuezilla team members are regular contributors to the Bugzilla mailing list/newsgroup. Issuezilla is not the primary focus of bug-tracking at tigris.org, however. Their Java-based bug-tracker, Scarab, a newfangled Java-based issue tracker, is under heavy development and looks promising!
URL: http://issuezilla.tigris.org/servlets/ProjectHome