www.redmine.org
Redmine is a flexible project management web application. Written using the Ruby on Rails framework, it is cross-platform and cross-database.
ProjectLibre :FIVESTAR:
ProjectLibre is an open source alternative to Microsoft Project. It is also the new updated version of OpenProj.
TaskJuggler :TWOSTAR:
A modern and powerful, Open Source project management tool. Its new approach to project planning and tracking is more flexible and superior to the commonly used Gantt chart editing tools.