JitterBug is a web based bug tracking system. It was originally developed by the Andrew Tridgell to handle bug tracking, problem reports and queries from Samba users. It is now available under the GNU General Public License in the hope that it will be useful for other projects.
JitterBug operates by receiving bug reports via email or a web form. Authenticated users can then reply to the message, move it between different categories or add notes to it. In some ways JitterBug is like a communal web based email system. This web page is itself a JitterBug page. You can get an idea of what JitterBug can do by playing with the various links and buttons. Some of the features of JitterBug include

Fully web based. The whole system is a single C program running as a CGI script.
HTML 3.2 compliant so it is usable from virtually any browser. No frames, Java or gratuitous graphics.
Uses your web servers built-in authentication
Built in SMTP mail client for sending replies and notifications
Each user can request email notification of changes to messages or directories
FAQ editing/creation built into the mail composing page
Tracking of replies and followups
Search and message selection controls
Each user can set personal preferences for colors, screen layout and personal details (like email address and signature)
Easily customizable
Doesn't require a database backend. All messages are kept as separate files
Users can add arbitrary notes to each message
An audit trail is kept for each message showing who did what and when.