Bugzero
Web-based bug, defect and issue tracking system in Java
Bugzilla
Open source web-based bug tracking system
Defect Manager
Bug and issue tracking system for Windows and the web
eTraxis
Open source web-based bug tracking system
expecco ALM
Web-based test and quality management platform (formerly expeccoNET)
Flyspray
Lightweight web-based bug tracking system in PHP