For implementing GTD you might try out this web-based application:

http://www.gtdagenda.com

You can use it to manage your goals, projects and tasks, set next actions and contexts, use checklists, schedules and a calendar.
A mobile version is available too.

As with the last update, now Gtdagenda has full Someday/Maybe functionality, you can easily move your tasks and projects between "Active", "Someday/Maybe" and "Archive". This will clear your mind, and will boost your productivity.

Hope you like it.