MantisBT is a free popular web-based bugtracking system. It is written in the PHP scripting language and works with MySQL, MS SQL, and PostgreSQL databases and a webserver. MantisBT can be installed on Windows, Linux, Mac OS, OS/2, and others. Almost any web browser should be able to function as a client.
Simple User Experience
Supports any platform that runs PHP (Windows, Linux, Mac, Solaris, AS400/i5, etc)
Available in 68 localizations.
Simple/Advanced Issue Pages
Multiple Projects per instance
Support for Projects, Sub-Projects, and Categories.
Users can have a different access level per project
My View Page
Search and Filter
Full Text Search
Shared Filters (across users / projects)
Built-in Reporting (reports / graphs)
Users can monitor specific issues
Attachments (can be saved on webserver or in database - can also backup to an FTP account)
Issue Change History
RSS Feeds (news, issues matching saved filters, issues matching a specific project)
Customizable issue workflow
Sponsorships Support - users are able to place bounties or sponsorships for specific issues, also developers can track such sponsorships / payments.