Bug-Buddy - A graphical bug reporting utility for GNOME. It allows users to easily make quality bug reports. - http://directory.fsf.org/bugbuddy.html
Trac - Trac is a web-based software project management and bug/issue tracking system. It provides an interface to Subversion and an integrated wiki. - http://trac.edgewall.org/
RT: Request Tracker - A free web and email-based bug tracking and trouble ticketing system. Features list, documentation screen shots, and download. - http://www.bestpractical.com/rt/
phpBugTracker - A web-based bug tracking system built in PHP. - http://phpbt.sourceforge.net/
Bug-A-Boo - An open source bug tracking system running on any web server allowing the execution of tdbengine binaries. It is dristributed unter the terms of the GPL. - http://www.bug-a-boo.org/
IssueTrackerProduct - Open-source web-based issue/bug tracker, written in Python. - http://www.issuetrackerproduct.com/
BugTracker.NET - Opensource web-based bug tracker written using ASP.NET and C#. - http://btnet.sourceforge.net
Bugzilla - Bug tracking used by the Mozilla projects. Inherently web-based, written in Perl, and uses MySQL as its database back-end. Open-Source. Updates available via CVS. - http://www.bugzilla.org/
Bugs Online - A free (GPL) bug tracking system for use with Microsoft IIS, Access and ASP. - http://bugsonline.sourceforge.net/
Roundup - A simple-to-use and -install issue-tracking system with command-line, web and e-mail interfaces. - http://roundup.sourceforge.net/
Teamatic - Free web-based bug tracking, ticketing and project resource tracking system. Designed to meet the requirements of most Information Technology, Consulting and Engineering projects. - http://www.teamatic.com/
Agstools - BugTrack, simple free bugtracking system based on perl5 and any DBI compliant database. - http://www.agstools.com/
WREQ - Work Request, a distributed request/problem tracking system, written in Perl 5. Access by Web or email. - http://www.math.duke.edu/~yu/wreq/
Mantis - php/MySQL/web based bugtracking system. - http://mantisbt.sourceforge.net/
JitterBug - A web based bug tracking system, originally developed by the Samba Team to handle bug tracking, problem reports and queries, now available under the GNU General Public License. Operates like a communal Web based email system - no database back end requir - http://samba.anu.edu.au/cgi-bin/jitterbug
Debian Bug Tracking Software - GPL'd Perl-5 scripts which maintain a database of problem reports. Input by email, listings via webserver. - http://www.chiark.greenend.org.uk/~ian/debbugs/
Keystone - Data stored in a SQL database, accessed by any Web browser which understands forms, tables, and cookies. By StoneKeep consulting. Free for under 10 users. - http://www.stonekeep.com/