Of all of the unexpected uses for my smart phone, using it as a level is my favorite.
When I first got my smart phone I loved the fact that it doubles as a flashlight: clever and really useful. Recently I’ve had some issues with my washing machine, and I’ve been thrilled that I can just pull out my phone and use it as a level.
Smart phones have built in tilt sensors (that’s how they know to switch the orientation of the screen, and allow pinball and other direction-sensitive games), so they can also be used as a level.
iOS 7 on the iPhone has a minimalist level in the Compass app–swipe to the left on the compass and it will appear. I prefer third party levels with a more traditional appearance–just search in the App Store or Google Play for “level” or “bubble level” to find one that’s right for you.