I'm surprised no one has commented on my previous post - I thought everyone knew what Mr. Project is (was).
From the old Mr. Project pages:
"MrProject is a project planning, scheduling and tracking tool for the GNOME Desktop aiming to act as a better replacement than available proprietary tools."
"MrProject is a project management tool that allows the user to manage several aspects of a project, including among others planning of tasks using Gantt charts and allocation of resources. It is released under GPL which means that you can use, modify and release the code at your will."
Somewhere along the way the software's name was changed to Planner. At some point a Windows port was created as well (the links in my previous post).