OS X: Apple's Reminders app is far from the most advanced to-do list app out there, but it's dead simple to use and syncs between all of Apple's devices. If you like the Mac version, Remind Me is a must-have add-on that lets you create new to-do items and reminders with just a few taps.
Remind Me lives in the menubar, but you're probably better off using a customisable keyboard shortcut to invoke it. Once you hit the shortcut, a small window will pop up that prompts you to choose a list and add a title, notes, reminder date, and due date. Hit save, and it will be added to the Reminders app and sync to your iDevices without ever futzing around with faux-leather trim.
Remind Me is about as simple as apps get, but it's a no-brainer free download if you've bought into Apple's take on the to-do list.