MantisBT is a web based open source issue tracker that provides a
delicate balance between simplicity and power.  Users are able to
get started in minutes and start managing their projects while
collaborating with their teammates and clients effectively.

It is written in the PHP scripting language and works with the MySQL
(or one of its forks), and PostgreSQL databases and a web server.
Almost any web browser should be able to function as a client.

WWW: http://www.mantisbt.org/