I use redmine for my current projects and love it. Feature wise, it's about the same with other similar products, but the UI is beautiful and highly usable. Very good bug, issue and source control integration.
What I like about it the most is its level of verbosity for issues. It asks just the right amount of information, and no more.
A good example of redmine is the official Ruby language project: http://redmine.ruby-lang.org/