I’ve been using a free GTD system called Tracks for about a month now.  From their web site:

Tracks is a web-based application to help you implement David Allen’s Getting Things Done™ methodology. It was built using Ruby on Rails, and comes with a built-in webserver (WEBrick), so that you can run it on your own computer if you like. It can be run on any platform on which Ruby can be installed, including Mac OS X, Windows XP and Linux. Tracks is Open Source, free and licensed under the GNU GPL.

So far I’m really diggin’ it. I installed it on an Ubuntu server in my office so I can always access it from my laptops or my iPhone. The application is very, very straightforward and does a good job of giving me the tools I need and none of the tools that I don’t. The mobile version (included) is also implemented very well, allowing you to enter tasks and projects quickly from your phone.

If you’ve been looking for a simple task management system you can run locally and access from many devices, Tracks might just be for you.