Bugtracking

Trac

FlySpray

Mantis

Phorum

Opentrack

Helpdesk

Bluetail

PHP Helpdesk

openTicket

Double Choco Latte

GNATS

ProManager

WebCall

PhpBugtracker

Mozilla

http://www.mozilla.org/projects/bugzilla/about.html

Scarab

http://scarab.tigris.org/

JitterBug

Request Tracker

Bug Tracker