Quality Control theme by spencerfinnell turns a basic WordPress installation into a simple and straight-forward ticketing system.
Quality Control allows users to create tickets, assign statuses, milestones, categories, and tags. Other users can then interact with the tickets, updating their state, milestone, or any other property.
Theme homepage and demo: http://getqualitycontrol.com/
Download: http://wordpress.org/extend/themes/quality-control