I have been trying out Vitalist for managing my projects and tasks for the last couple of days. It's a web-based system for implementing GTD, it's simple and it's free! No download needed: simply access the web.
It even has a mobile edition: I easily access all my projects/next action on my mobile.
What more could you ask for. Go ahead and have a look:
Would like to emphasise that I have nothing to do with the company/don't even know anything about it: I simply fell in love with the simplicity and the clean user interface.