AstroImageJ
http://www.astro.louisville.edu/software/astroimagej/
"Munipack is a free open source tool for processing of astronomical images"
http://munipack.physics.muni.cz/
"Lxnstack is a program designed to align and stack astronomical images (both planetary and deep-sky)"
https://sites.google.com/site/lxnstack/home