I have used the built in Compass App a couple of time on my iPhone, but recently I came across a feature of that App I was not aware of. The Compass App has a built in level!
If you have iOS 7 or later on your phone, then open the Compass App, then once you see the compass on the display, swipe the screen left. That will bring up a level.
This isn’t something that I will use every day, but it adds one more feature to my iPhone.
See my other iOS articles