Mantis Bug Tracker (MantisBT) is a Web-based issue tracking system that is both simple to use and highly customisable. A plugin system allows users to easily create and add new features. MantisBT is over 10 years old and is actively developed.
Originally developed for the Psi project, Flyspray is an
easy to use bug tracking system for those who don't
require all the complexities of something like Bugzilla.
It supports multiple users, file attachments, and Jabber