iOS: There's no shortage of note-taking apps on the iPhone. If you're looking for something incredibly simple that syncs with services including Dropbox, Evernote, Twitter, Facebook and email, then Squarespace Note is worth a look.
When you open Squarespace Note, you're taken directly to a blank page to type a note. Once completed, you can swipe up to save the note to your phone, swipe right to send it elsewhere, or swipe left to look at your old notes.
Squarespace Note doesn't have quite the range of its rival Drafts, but it does cover the major services (and its parent company Squarespace).
It's very simple, but if you want your note-taking app to act like a sheet of scratch paper on your desk, and not a full-fledged tool, Squarespace Note is worth a look.