Are you looking for hosted or non-hosted? We're using redmine, an alternative to Trac, which was a breeze to set-up and use. The UI is simple and intuitive. Quite flexible with configuration, allows multiple projects, Source Code Management integration (for SVN, git, CVS, all the common ones), integrated wiki, forums, files, etc. Other names to toss around include unfuddle, jira, lighthouse, and sifter app.
