Here's a simple, yet handy, tip for you iOS 7 users out there. Did you know that iOS 7 has a built-in level? True story. It's accessible from within the compass app. Simply open the compass app and swipe to the left to reveal the level. With the level active, place the device on a flat surface to determine if it's level or not.
This utility is great for fixing wobbly tables, laying tile or wood floors and even hanging photos on the wall.