Most online source code hosting services (such as Github, Google Code, BitBucket, Launchpad, etc.) offer bug tracking (aka "issue tracking") support. However, this page is specifically about free software bug trackers implemented in Python that you can install on your own systems.
Bugs Everywhere -- http://bugseverywhere.org/
DITrack -- http://www.ditrack.org/
git-issues -- http://github.com/jwiegley/git-issues/tree/master
Roundup -- http://roundup.sourceforge.net/
trac -- http://trac.edgewall.org/
TODO: make IssueTracking point to here