Of the many to-do apps I've tried, the one I actually use is UnTodos.com, with the somewhat loose categorization of tasks into "Today", "Soon" and "Whenever", and easily moving tasks about, it's one that I haven't given up on (vs. ta-da, rememberthemilk, etc.)

And I also find PivotalTracker.com really usable. At first the terminology (story, icebox...) was a little uncomfortable for me, but I keep coming back to it to handle projects, along with co-workers.