iOS: The iPhone has a ton of great to-do apps, most of which will cost you a few bucks. One of its best, TeuxDeux, is available for free right now.
TeuxDeux isn't our favourite to-do app (that honour goes to Orchestra, with Wunderlist in second place), but it's a great option if the other to-do apps are just too complicated for you. TeuxDeux is simplicity and minimalism at its finest, letting you add tasks, check them off, and not worry about anything else -- though there is a handy "Someday" folder for items that aren't too urgent.
If you're still on the hunt for the perfect to-do app, the regularly $2.99 TeuxDeux is worth grabbing while it's free.