Part of the U.S. Department of Energy, the EIA "collects, analyzes, and disseminates independent and impartial energy information to promote sound policymaking, efficient markets, and public understanding of energy and its interaction with the economy and the environment." (source: website)
Covers: coal, electricity, natural gas, petroleum & other liquids, nuclear & uranium, renewable & alternative fuels, total energy, consumption, efficiency. Features energy analysis &projections, financial market analysis & financial data for major energy companies, and environmental impact data (greenhouse gas data, electric power plant emissions, voluntary reporting).
Also, see the EIA's Open Data tools.
From the U.S. Department of Energy. Access energy datasets, search and developer tools, and the Public Data Listing for the Department of Energy. (The Public Data Listing is a machine-readable list of all of the publicly available datasets maintained by the Department and its program and staff offices).