rt3, or Request Tracker (Score 1)

I'm a big fan of rt3 ( http://bestpractical.com/rt ).

Pros:
- Free and open source
- Web, email, and command line interface
- Flexible (I've used for bug tracking, support departments, task and project management, documentation, mailing lists, and more)
- Scalable (I've seen it go beyond 100,000 tickets on an average server with no performance hit)
- Excellent community (mailing lists, wiki, book, contrib section on the site)

Cons:
- Written in Perl, so its a bit tricky to configure and customize

Give it a try.