Issue / Bug Tracking / Ticket Systems

Bug Tracking – Issue Tracking – Ticket systems

Open Source

Redmine

Trac

The Bug Genie

Flyspray – The Bug Killer!

osTicket:: Open Source Support Ticket System

The Bug Genie – friendly issue tracking

Trellis Desk.

Main Page – Cerberus Helpdesk Wiki
Also have a paid version.

Commercial

Axosoft OnTime
My personal favourite windows-native Bug/Project tracking system.

Jira
Very good by all accounts.

Windows / Desktop

Jumpbox have a good selection of these systems available as a single download + install.
Redmine, Bugzilla, Mantis, Trac, OTRS etc etc