ImageJ is free and easy to use image analysis software funded by we the taxpayer. Once you calibrate the image to a known scale you can measure anything on the picture as long as its in the same focal plane as the scale. So if you have a ruler in the picture you can use that as your calibration, then draw just about any shape and compute length, area, angle... all kinds of schtuff

http://rsbweb.nih.gov/ij/download.html

If you go this route, let me know if you need some help setting up your first calibration.