A nice GTD system
RTM (Remember The Milk) is a free online task management system.
This blog post http://blog.rememberthemilk.com/2008/05/guest-post-advanced-gtd-with-remember-the-milk/ shows how RTM can be tweeked to work as a GTD system.
For outlook fans, the FREE “SprynxCRM Outlook sync For Remember The Milk” http://www.gassumo.com/Home/sprinxcrm-outlooksync-for-remember-the-milk/download will help you use RTM from within Outlook (supports 2007 and 2010).
Emad's Blog 