FogBugz

A popular commercial bug tracker, which even gained a web interface and integrated wiki over time (I wonder where they got the idea). Covers more or less the same ground as Trac, see their ​feature list for inspiration.

Some people are even using FogBugz and Trac together, the former as a BTS and Trac for the Wiki and Subversion browser, see ​TracFogbugz.