Image processing software

Multispec	https://engineering.purdue.edu/~biehl/MultiSpec/
ImageJ		http://rsb.info.nih.gov/ij/

GIS software

Grass		http://grass.fbk.eu/
QGIS		http://www.qgis.org/

Programming environments

GNU Octave	http://www.gnu.org/software/octave/
R		http://www.r-project.org/

Data plotting

Veusz		http://home.gna.org/veusz/
Plot (MacOS)	http://plot.micw.eu/

Catalogues of satellite data

http://earth.esa.int/dataproducts/
http://www.sat.dundee.ac.uk/
http://glcfapp.glcf.umd.edu:8080/esdi/index.jsp
http://reverb.echo.nasa.gov/reverb/
http://earthexplorer.usgs.gov/
http://catalog.scanex.ru/dewb/step1.pl
http://disc.sci.gsfc.nasa.gov/giovanni/overview/index.html
http://www.ersdac.or.jp/eng/index.E.html
http://airs.jpl.nasa.gov/data/get_AIRS_data/ - AIRS
http://www.ncdc.noaa.gov/oa/satellite.html - US National Climate Data Centre

Satellite data galleries

http://earth.eo.esa.int/satelliteimages/
http://visibleearth.nasa.gov/
http://earth.jsc.nasa.gov/
http://www.jpl.nasa.gov/radar/sircxsar/ - SIR-C/XSAR SAR images
http://earthobservatory.nasa.gov/

Reference data

http://rst.gsfc.nasa.gov/ - tutorial
http://www.ceos.org/ - committee on earth observation satellites
http://rsd.gsfc.nasa.gov/rsd/RemoteSensing.html - tutorial
http://speclab.cr.usgs.gov/browse.html - reflectance spectra
http://www.icess.ucsb.edu/modis/EMIS/html/em.html - TIR emissivities
http://www.n2yo.com - real time satellite tracking