B.1. Bugzilla

Bugzilla is very popular, actively maintained, and seems to make its users pretty happy. I've been using a modified variant of it in my work for four years now, and like it. It's not highly customizeable, but in a odd way that may be one of its features: Bugzilla installations tend to look pretty much the same wherever they are found, which means many developers are already accustomed to its interface and will feel they are in familiar territory.

http://www.bugzilla.org/