Sometimes when taking a photo, you want to line up the objects in the shot on a perfectly straight plane. Most standalone cameras and even iOS 7's built-in camera app use gridlines to help you line up your shot, but you have to eyeball it to make sure the shot is level. If you want a level line in your camera app, then you should check out LVL Cam.
LVL Cam is a lightweight camera app that adds a level line to the camera interface. The app finds the center of the frame and places a circle right in the center. You must move your phone and tilt it so the level lines up with this center circle. Once you are lined up, you can tap the shutter button to snap a shot with the focus on this center point.
The app has a few bonus features that allow you to line up your photos and set a different focus point, and a handy ghosting feature that helps you take two photos that are lined up exactly the same. There's also an auto-shutter option that'll automatically snap a photo once you lineup the level. This last feature is a lifesaver if you are using your iPhone camera without a tripod. With unsteady hands, it is difficult to keep a shot lined up perfectly while you fumble to find the shutter button.
LVL Cam is available for US$0.99 in the iOS App Store.