Roundup is a simple-to-use and install issue-tracking system with web, command-line xmlrpc, rest (in the 2.0 development line) and e-mail interfaces. It is based on the winning design from Ka-Ping Yee in the Software Carpentry Track design competition. Downloading the source allows you to start a demo instance in a few minutes to see if it will fill your needs.
Eventum is a user-friendly and flexible issue tracking system that can be used by a support department to track incoming technical support requests or by a software development team to quickly organize tasks and bugs. Eventum is used by the MySQL AB Technical Support team, and has allowed them to dramatically improve response times.
Trac is a software project and release management tool, and source control frontend for Git and Subversion. It provides issue tracking, timelines, progress/roadmap tracking and documentation features (Wiki with MarkDown support).