Open Source

Achievo
A web-based project and resource management tool for business environments. The site includes product support, a demo, and a development blog. (PHP,MySQL)
http://www.achievo.org

dotproject
BSD-licensed PHP-MySQL web-based project management application, with Gantt chart support.
http://www.dotproject.net/

PHProjekt
Web-based groupware tool which includes calendar, project-management, time-card management, PDA synchronization, email, voting system and personal to-do list management.
http://www.phprojekt.com

Project Management Interface
Web-based project management application. Provides portal management to a large number of coarsely-managed projects.
http://majordojo.com/pmi/

Project Open
Web-based project resource planning system, with project rooms, and tools for managing clients, invoices, time, and cost. Designed for the advertising, IT consulting, translation and knowledge management sectors.
http://www.project-open.com/

Project Pier
An application for managing tasks, projects and teams through an intuitive web interface. Documentation, community, blog, and download pages are provided. (PHP5)
http://www.projectpier.org/

Redmine
A flexible project management web application which includes a gantt chart, calendar, wiki, forums, multiple roles, and email notification. Information on the project, a demo, download information, and forums are provided. (Ruby)
http://www.redmine.org/

Trac
Project management and bug/issue tracking system. Provides an interface to Subversion and an integrated wiki.
http://trac.edgewall.org/

Track+
Issue tracking system. Web based tool for bug, issue, and task tracking. Offers Gantt charts, diagrams, configurable workflows, email reminders of due tasks and PDF reports.
http://www.trackplus.com/

TUTOS Groupware Suite
Web-based groupware suite.
http://www.tutos.org

WebCollab
A web-based system for projects and project management.
http://webcollab.sourceforge.net/