Stuff is a free GTD application for Mac OS X. It's a project not only by me, but also by MensWork. It does basically the same what The Hit List or Things does, but it should be free and maybe it will be better.
You can contribute on the Stuff sourceforge site or on its own site, stuffapp.bplaced.net. We would be pleased about a new combatant!
Stuff is not ready yet, but here is a screenshot of what it will look like when it's finished:
The idea behind this project is simple: There is no good GTD app for Mac under 20€. And because I'm a student and I don't want to spend about 50€ on a GTD program, I've decided to create my own application.