Ask HN: Best daily “get-stuff-done”/todo app?

I have a bad habit of getting off on tangents when I'm trying to get my work done each day. The tangents relate to my work, but they go off into more "research-level" depth on a subject than is really necessary to just get the work done (as a chemical engineer, you wouldn't believe how much time I've spent teaching myself group theory).

I've been reading up on this problem, and some studies indicate that making a todo list of manageable action items for each day results in modest productivity gains.

What's a good app for this? I've tried using a paper notebook, but I always lose it or forget to mark things off.

I need something that is either a mobile app or runs on OS X. In contrast with the myriad todo apps that already exist, this one needs to have task completion as its primary objective; i.e., it needs to prod me to stay on task and get things done — incentivize completion.

What all is out there?