Web: GTDNext is a to-do list webapp that makes it easy to manage your tasks based on the principles of the Getting Things Done (GTD) productivity method, especially if you're new to it.
David Allen, the creator of the GTD technique, says you need to prioritize your task list to successfully finish it. His method involves five steps: Capture, Clarify, Organize, Review, Do. GTDNext is all about going through those five steps with a to-do list.
Capture: Add projects and actions to your list, as and when they occur.
Clarify: Break your task into smaller sub-tasks. The first task or sub-task shows up in the "Next Actions" tab.
Organize: Put your tasks in the right order by dragging and dropping.
Review: In the "Next Actions" tab, go through all the items you need to do. Mark the ones you want to tackle immediately by clicking the 'Focus' icon.
Do: Switch to your Focus list. Start doing.
This video demonstrates the prioritization process. The app also has the usual features of task apps, including scheduling tasks, adding tags, and so on. The free account limits you to four areas and 200 projects or actions. The premium account ($4/month) has no restrictions and lets you archive and restore projects or actions. The only thing missing is a good mobile app.