A Level: Virtual Spirit Level for iPhone

From Wired -

A-Level is a level simulator app that uses the iPhone's motion detectors to determine whether it is flat. If not, a numerical readout tells you how many degrees off you are and the app also shows the traditional bubble-in-liquid display. You can use it in landscape or portrait mode, as seen above, or laid flat on its back.

As for accuracy, it comes with a disclaimer: "This application is for recreational use only". A promised update will allow calibration, which should help. But for now, just having a level in your pocket all the time is great, and means one less thing in the toolbox.

Source article here.