Android: Floating Stickies is a great app for creating customisable sticky notes that can float above anything else on your screen.
Clicking on the app's icon will open up your saved stickies, or create a new one if none exist. You can add text, resize and drag the stickies around to your heart's content, and they will always float on the top of your screen. If you want to move a sticky out of the way, dragging it to the left side of the screen will replace it with a tiny translucent pencil icon, which you can drag back onto the screen to restore the note. To save your stickies and quit the app entirely, just tap the message it creates in the notification drawer.
Obviously, this isn't going to replace an app like Evernote for mobile note-taking, but it's a great intermediate step for quick idea capture, especially if you need to refer to information on the web or within another app. Floating Stickies is free on Google Play.