On reflection, you probably deserve a more complete answer.
Bugzilla is the free open-source bugtracking system that the Mozilla people have published. And when I say open-source, I mean that the bugzilla software is open-source, not that your programs need to be open source.
It's what the Empeg guys use for their bug tracking.
I only interact with it from the beta tester's perspective, but I can attest that it seems to have a lot of useful features from that point of view. Entering a new bug is painless and straightforward. Searching for existing bugs is not painless, but it's straightforward and very complete. You can search on all sorts of fields and criteria. I figure that the complexity of the search form is necessary for it to be so powerful.
Rob would have to chime in and say whether it works well from the project manager's perspective. He's able to customize the user interface pretty well, but I don't know how much management overhead there is to keep it running.