I'm a big fan of Wunderlist. It's an amazingly simple and powerful to-do app that syncs effortlessly between all my devices. And now they are making it better. Adding folders is a big improvement. I currently start all my lists with a prefix (for example, SBB Blog Post Ideas) so I can keep all my similar lists together. But that gets clumsy. So having folders is a big improvement.
Some of the other additions are great, too. Quick Add looks great. And an API to integrate with other apps has the potential to integrate Wunderlist throughout my workflows.