iOS: You have lots of options for notes apps on the iPhone. If you're looking for something fast that's good for just jotting down a quick note, Noted is a free and simple solution.
Noted is similar to an app like Clear, where it's all about gestures to navigate the app instead of buttons. You pull down to create a new note, swipe left and right to flip between notes, pinch to get back to the default card stack view, and use a two-finger swipe to delete a note.
From there, Noted is basically a place to drop a note quickly. It loads fast and doesn't have a lot of options, but if you just need a place to quickly jot things down, Noted is a great app for doing it.