Definitely Bugzilla ... I've quested for years for the ultimate bug system and Bugzilla is the closest I've come to being satisfied. It's ugly out of box, but a little spit and polish and you can make it look nice (and if you don't care about the looks, it just plain works). Installation is pretty easy (compared to some other open source projects), but you will need to know a little about unix/linux/mysql to do it. I've used it both as an enduser as my primary bug system at work for the past 3 years and as an enduser through the Empeg bug system and both are very nice.

oh yeah and it's free.

ms