Some of NASA's and ESA's data processing software is also open source (although the data to make them useful is often not free).
A couple examples are:
SeaDAS (SeaWiFS Data Analysis System), developed by NASA for processing Ocean Colour (not necessarily visible colour) data (originally for the SeaWiFS sensor, but other sensors such as MODIS have been added since). http://oceancolor.gsfc.nasa.gov/seadas/
The BEAM Toolbox (Basic ERS&ENVISAT (A)ATSR and MERIS Toolbox), developed by Brockmann Consulting for ESA to process data from the MERIS, ERS ATSR, AATSR, ASAR, and MODIS Ocean Colour sensors. http://www.brockmann-consult.de/beam/