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. 
FIJI is the updated version of ImageJ that includes all of the processing plugins prepackaged.
Download: https://imagej.net/software/fiji/downloadsl
About the software: https://imagej.net/software/fiji/