Jira by Atlassian on our own server, but you can also have them host for a very modest amount for a small number of users. We also have it integrated in to SVN by creating a post-commit hook that sends an Email to Jira; the SVN log then gets posted to the Jira when the user supplies the Jira ticket number. I've used many bugtracking systems (haven't used Bugzilla, though), and wrote a couple of my own, and found Jira to be very useful and easy to customize.

Back in the dark times, 1980, I played a CLI-based Star Trek game on a multi-user PDP11. You would string commands together to move to a location/fire on an enemy/run away.