Flyspray is an uncomplicated, web-based bug tracking system written in PHP for assisting with software development. It was originally conceived when the Psi Jabber client project couldn't find a bugtracker that suited their needs, and has been made available for everyone to use for their own projects.
Multiple database support, currently MySQL and PGSQL
Easy to use
'Watching' tasks, with notification of changes
Comprehensive task history
Advanced search features (though easy to use)
Two syntax options for task descriptions and more (Dokuwiki / plain text)