If you need a defect tracking system, Mantis is open source, written in PHP and works with MySQL. It is very nice! I installed it for a client this weekend and I really like it. It even has RSS feeds!
It is very customizable as well. Especially if you know a little bit about PHP.
If you need defect tracking, and you like FREE – look at Mantis!
Mantis is a free popular web-based bugtracking system (feature list). It is written in the PHP scripting language and works with MySQL, MS SQL, and PostgreSQL databases and a webserver. Mantis has been installed on Windows, Linux, Mac OS, OS/2, and others. Almost any web browser should be able to function as a client. It is released under the terms of the GNU General Public License (GPL).