Bug Tracking
A quick Google Search reveals many options for defect tracking systems.
The Opensourcetesting website (deserves it's own article on Quality Assurance with links to http://www.testmanagement.com/ ) offers a rather complete list of bug tracking databases: http://opensourcetesting.org/bugdb.php
And Karl Fogel's book on Producing OSS (is undoubtedly a worthwhile read) has an appendix listing bug trackers http://producingoss.com/html-chunk/bug-trackers.html
This page is an editorial + review of many categories of FOSS software from Bug Tracking to Support Desk solutions (perhaps dated).
There are also many listings over at the DMOZ page on Bug Tracking
One of the old standby's of Bug Tracking is 'RT' or Request Tracker. By now, it's up to version 3.4.5
One PHP product is called 'Workbench' Meanwhile, another is called RUQueue (written at Rutger's University as a PHP version of RT)
More PHP options include the Whups system from the Horde. and CodeTrack and FlySpray doesn't look too bad
For Python, there is Roundup and also IssueTracker which is built on Zope
