Open Source Remote Sensing
Within the realm of remote sensing, little is available for free. The discipline is fairly esoteric in both development and academic exposure thus the potential user community is fairly small. However, a couple options do exist.
Quantum GIS (QGIS) is a user friendly Open Source Geographic Information System (GIS) licensed under the GNU General Public License. QGIS is an official project of the Open Source Geospatial Foundation (OSGeo). It runs on Linux, Unix, Mac OSX, Windows and Android and supports numerous vector, raster, and database formats and functionalities.
The System for Automated Geoscientific Analyses (SAGA) was developed by a team working at the University of Hamburg; Hamburg, Germany. The software grew from a need to analyze raster data, specifically digital elevation models (DEM).