Bugzilla is the leading open-source/free software bug tracking system, with high-profile installations at mozilla.org, GNOME, Red Hat, and NASA, among others. It features a comprehensive set of fields, attachment management, inter-bug dependencies, email notification of changes, a powerful query interface, reporting, and Web, XML, email, and console interfaces. It can be managed entirely using a Web-based UI. The user UI is customizable using templates.
AccuRev is a fast, powerful, and easy-to-use Software Configuration Management (SCM) tool with integrated issue tracking based on "Streams" that more efficiently manages complex parallel and global development. Streams are a combination and superset of branches and labels that are first class objects instead of being file-by-file attributes. AccuRev records all merges and patches, so you never have to re-merge the same code. It supports file and directory renaming and merging, follows a client/server model, and is transaction-based. It includes an integrated graphical 3-way diff and merge tool. AccuRev provides an Open API (and SDK) to integrate with any third-party development tool (IBM/Rational ClearQuest, Serena TeamTrack, and more out of the box). It also integrates with Eclipse, the MS SCC API (MS Visual Studio), Sun Java Studio, any diff/merge tool with a command line interface, and more.
The Bluetail Ticket Tracker (BTT) is a system for creating, storing, and keeping track of tickets. A ticket describes an issue of some sort. It can, for example, be a Trouble Report or a ToDo note. BTT also makes it possible to store customer information, which can be coupled to a ticket. BTT comes with a wiki-style knowledge base and an automatic mail store.