Done.io (http://done.io) is yet another task manager (it was previously called pat.io and was reviewed here ~1 year ago). Since then it's gotten a full API, a way better UI and name, and some needed functionality.
It introduces the idea of "shifts", items that you need to do but don't want to worry about right now. I find it works very well as an adjunct to other, more full-featured systems, as my "stuff I really need to worry about right now" list.
Enjoy, and thanks.