Recs.
Updated
GTDNext is a task management app available online and works in Chrome, IE and Firefox. GTDNext focuses on more than simple to-do lists. GTDNext is purpose built to implement GTD.
SpecsUpdate
Pros
Pro Automatic next actions
When it comes to choosing your next action, most to-do apps suffer from the same problem: All of your actions (sometimes hundreds in total) are visible to choose from. This can overwhelm users and cause them to give up on the system. GTDNext is different. In the "Next Action" view, only the very next action is shown for each of your projects. This results in a nicely consolidated view of only the things that need your immediate attention.
Pro Areas of Focus
GTDNext supports the “Areas of Focus” methodology from Getting Things Done. You can filter all of your tasks to just show your work tasks, side project tasks, or personal tasks. There is also a global filter, so that when you’re hard at work you can focus only on your work projects and tasks.
Cons
Con Web only
GTDNext does not have native apps for other devices, but it does work across PC/Mac/Linux on the web client.