Freeware

ViewFinder V2.1 by EDRAS — Provides basic image viewing and manipulation capabilities
ArcExplorer by ESRI — Lightweight GIS data viewer that offers an easy way to perform a variety of GIS functions
Terrain Analysis System — A stand-alone GIS package that accepts raster and vector formats and possess many standard spatial analysis functions
GRASS — A powerful open source GIS used for geospatial data management and analysis, image processing, graphics/ map production, spatial modeling, and visualization
Google Earth by Google — Its the universe inside your PC
EasyGPS — Easy to use software for transferring data between your GPS and computer
DIVA-GIS — A GIS particularly useful for mapping and analyzing biodiversity data
MultiSpec — A processing system for interactively analyzing Earth observational multispectral and hyperspectral image data
ENVI — (evaluation version) Software to quickly, easily, and accurately extract information from geospatial imagery
MapViewer by Golden Software — (demo) Software for thematic mapping and spatial analysis
USGS Digital Data Viewer: dlgv32 PRO — A limited-feature version of the Global Mapper commercial software
Wikipedia list of GIS software — Lists, describes, and links to free open source and commercial GIS software
QGIS — A free GIS software
ConnectED — Free software for K-12 educators.