The ImageJ software is a powerful freely available tool for image processing and analysis. It is developed and maintained by the National Institutes of Health (NIH). One of its most important benefits is the fact that it is cross platform as it was developed in Java. In addition ImageJ also offers a very active developers community and a very very easy way to plug-in new algorithms and tools.
Some interesting papers and tutorials about ImageJ are listed below:

• NIH, The ImageJ documentation, 2014
• J.Y. Ruzicka, An introduction to ImageJ, 2014
• Automatic determination of particle size distribution
• Dune Sciences, Inc. Particle size analysis of nanomaterials using ImageJ, 2011
• N.H. Viet, Analyze particles by using ImageJ, 2010