ImageJ is an open-source software (free download from NIH). It can display, edit, analyze, and process images; for example, it can calculate area and pixel value statistics of user-defined selections in images. 
Download: https://imagej.nih.gov/ij/download.html
About the software: https://imagej.nih.gov/ij/index.html
Fuji is a processing package based on ImageJ, prepackaged with many useful plugins.
About the software: http://fiji.sc