Bugzilla is the leading open-source/free software bug tracking system, with high-profile installations at mozilla.org (250,000 entries), Gnome, Red Hat and NASA, among others. It features a comprehensive set of fields, attachment management, inter-bug dependencies, email notification of changes, a powerful query interface, reporting, and web, xml, email and console interfaces. It can be managed entirely using a web-based UI. The user UI is customisable using templates.
Bugzilla is developing rapidly, and has a strong developer and support community in its newsgroup and IRC channel.
Bugzilla also has comprehensive documentation.
Bugzilla is a server application, you must be using your Mac OS X box as a webserver to make use of it.