Its a brilliant new feature of iOS 7, yet you probably wont have heard about it virtually anywhere. There is a spirit level (a fantastically accurate one that works on multiple planes) built in to the compass app.
Open the compass app and then swipe left (thumb on right of screen and swipe across to the left) to go to the next screen
What you get is this – in this mode the level checks if your phone is flat on its back. Set your phone on your desk and you can see if it is level! You may hope its not off by 9 degrees like the screen-shot below or no pen or pencil would ever stay still.
If your desk is level – it goes green 🙂
If you turn your phone on its edge, the display changes to allow you to check the level across the phone. You can make this even more accurate for longer distances by putting the phone on a plank of wood, and it is remarkably accurate.
The same as with the phone flat, if you get it level, the screen goes green! No more paintings hung at an angle for iPhone users!