I can't believe no one has mentioned Jira. It beats the tar out of bugzilla, trac, and mantis. It's not free, but it's by far the best product I've used, with

I can't believe no one has mentioned Jira. It beats the tar out of bugzilla, trac, and mantis. It's not free, but it's by far the best product I've used, with the largest plugin development community, and a great list of sibling applications you may want to try later on. It's not open source, but you do get the source code when you buy an enterprise license. Jira is also great for managing project work internally.