iProtractor is a protractor that measures angles and pixel distances on your photos. Select an image from your photo library (iPhone & iPod Touch), or take a new one with your camera (iPhone only). Touch & drag the "lenses" to align the edges of the protractor with lines in your selected image, and the protractor gives you the angle made by the edges. It also gives you the length (in pixels) between the lens crosshairs.
Choose whether you'd like your angle reported in degrees (decimal), degrees (arcminutes), radians, and multiples of π.
Most useful, iProtractor has a "four-point" mode, as illustrated in the second screen shot, for use when finding the center of the angle is inconvenient or off-screen, a feature not available in other "protractor" apps.