Trimble
http://www.trimble.com/

Additional background information on the GPS system.
http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

Information on Ag-Chem, Lor*Al, and Soilteq.
http://www.agchem.com/

Wilco Farmers, Inc.
http://www.wilcofarmers.com/

Ag Leader Equipment
http://www.agleader.com

Precision Farming Enterprises
http://www.precisionfarming.org

Farmworks Software
http://www.farmworks.com