Programs, Plugins, and Extensions:

Google Earth: An easy-to-use GIS program from Google with basic functions.

NASA World Wind: Similar to Google Earth, using NASA sattelite images.

Geographic Resources Analysis Support System (GRASS): Free, open-source GIS progam.

Gstat: Open-source program for geostatistical modelling, prediction, and simulation.

Variowin: Free 2D spatial data analysis program.

ET Spatial Techniques: Downloads of editing tools for use in ArcGIS.

Hawths Tools: Plugin tools for ArcGIS.

X Tools Pro: Powerful extension for ArcGIS.

Data East: Online GIS store offering plugins and tools for use with ArcGIS.

VB Script Functions: Plugin to aid in data analysis in ArcGIS

CrimeStat Spatial Statistics: Powerful cluster analysis tools

MAPublisher: GIS program for use in Adobe Illustrator

Data:

National Geospatial Intelligence Agency: Downloads of DEM, Raster, and other files for use in ArcGIS.

National Cartography and Geospatial Center (NCGC)

GeoCommunity: Downloads of DRG, DEM, DOQ, and other files for use in GIS programs.

Land Process Distribued Active Archive Center: Downloads of a wide-range of NASA data.

Global Land Cover Facility: Downloads of remotely sensed satellite data.

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER): From the NASA Jet Propulsion Laboratory at the California Institute of Technology.

Geoeye: High-resolution satellite (IKONOS) photgraphy

The Mesoamerican Regional Visualization and Monitoring System (MesoStor): Offers on-line data delivery of digital maps across Central America.

The CGIAR Consortium for Spatial Information (CGIAR-CSI): Downloads of SRTM 90-meter DEM worldwide.

USGS Global Visualization: Sattelite imagery worldwide.