I've used Bugzilla, Mantis and Jira. Jira, while not free, is the best of those three for sure. They have a nice $10 for 10 user license that is quite afforda

I've used Bugzilla, Mantis and Jira. Jira, while not free, is the best of those three for sure. They have a nice $10 for 10 user license that is quite affordable and besides the 10 user limit, has no functional limitations. I've read good things about FogBugz but have not used it myself.