National Agricultural Statistical Service (NASS)
Easy to navigate, current information including news releases.
National Agricultural Library (USDA)
Useful links for agricultural research, includes an agricultural thesaurus.
Science.gov: Agriculture and Food
A comprehensive list of sites to search through.
Office of the Chief Economist (USDA)
Economic side of agricultural supply and demand.
The Core Historical Literature of Agriculture
Actual agricultural texts published over almost 200 years.