Mac: While we love Wunderlist for managing to-do lists, sometimes you just want to take yours back to the basics. Nitro offers some added simplicity in its interface and also syncs with Dropbox (and Ubuntu One), keeping your data somewhere that you can always access it.
If all you need to do is create a to-do list, Nitro makes that very easy. Just add tasks and check them off when you're done. If you like to tag and prioritise, you can do that too, and you can save tasks for a far-off date. It's a very simple approach to your tasks, covers all the basic features you ought to have in a to-do list app (without extra fluff), and you can download it for free.