TiddlyWiki producted the best demonstration of a Getting Things Done methodology (organize things to be actionable, display things under a given context, organize things by blockers) I've ever seen.

http://mgsd.tiddlyspot.com/demo3.html

This provides so many more features & organization than what a traditional Todo app provides.

The UX was great, but I didn't like the local data-lock-in due to tiddlywiki. ( Also - the whole ticklers (notification) system was a bit clunky. )

Once in a while I get the motivation to reproduce this as a full-fledged web or mobile app.