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Trac - Trac is a project management and bug/issue tracking system. Provides an interface to Subversion and an integrated wiki.
16bugs - 16bugs is an all-in-one bug tracking software that makes it easier for you to manage all sorts of bugs in performance with the simple interface; ultimately, you will be more productive for sure.
Bandcamp Artists - The Bandcamp app for artists and labels
Traq - Traq is a PHP powered project manager and issue tracker featuring multiple projects, milestones...
Mantis Bug Tracker - Mantis Bug Tracker is a free and open source, web-based bug tracking system.