ImageMeter is an Android app that lets you annotate dimensions and angles in your images with ease. What is special about ImageMeter is that it enables you to measure within the image long after you took the picture.It often happens that after taking the picture, being back at home or in the office, one realizes that some important distances have not been measured. This is no problem anymore with ImageMeter, because it enables you to measure the missing data directly from the images, afterwards. Additionally, with this feature, you can also easily add dimensions for places which are too difficult to reach. ImageMeter will take care of all perspective foreshortening and still measure correctly.
The complete manual is on its own page (but a bit outdated).