I love Trello, simply because it's such a generalized way to organize thoughts. I use it for everything from project outlines to detailed to-do, to-read and to-watch lists.

One thing I've found convenient for my high priority to-do card is a script that prints it on to my desktop. Trello already has a fairly solid API.

In my opinion the biggest challenge for to-do solutions is how to stay in the users face all the time. Which is probably why sticky notes on the monitor are still popular.