Roundup is just an amazing bug tracking system. It really is way more general then that, their own words for it are not bad: an issue tracking system for knowledge workers.

Why I love it:

• It's written in python
• It has a poweful and flexible concept/abstraction of a database very well suited for it's purpose.
• It has a very flexible page rendering and templating system. With clever URL crafting you can modify how and what the page displays.
• It's configuration is done by creating python objects (modifying the database layout) and writing python functions (automating and policing changes to the issues).

And lots lots more...

Obviously nothing is perfect and it still has a few minor bugs and annoyances. But this is just so much nicer and more flexible then say bugzilla.