Trade Associations:

American Gas Association represents local natural gas utilities that deliver gas to 54 million homes and businesses in all fifty states.
Natural Gas Supply Association represents integrated and independent companies that produce and market domestic natural gas.

Yahoo Sites:

Business and Economy: Companies: Energy: Petroleum: Natural Gas.
Science: Energy: Petroleum: Natural Gas.

Regulation: Different aspects of gas production and sale are regulated by different agencies. See specific aspect pages.

See also the See National Association of Regulatory Utility Commissioners for state links.

Government information:

Energy Information Administration:(home page)
--Natural Gas Menu includes the following, and a lot more:

U.S. Crude Oil, Natural Gas, and Natural Gas Liquids Reserves -- 1996 Annual Report.
National and State estimates of proved reserves of crude oil, natural gas, and natural gas liquids in the United States.
Natural Gas Annual
Natural Gas Monthly
Oil and Gas Field Code Master List - listing of U.S. oil and gas field names and location.

American Gas Magazine covers gas transport and distribution to users.
Competitive Utility -- policy analysis.
Energy Insight is a daily web news and document source. Free trial subscription.
Energy Technology Data Exchange - an international consortium, owns the Energy Database, with over 3 million bibliographic citations of international energy research and development information.
Pennwell has many books, directories and maps.

Other sources:

The Gas Research Institute does research on gas supply and use issues.
The Institute of Gas Technology looks at technology aspects of gas production and use.
Natural Gas Information and Educational Resources is an industry-wide collaborative effort to educate the general public on natural gas without company-specific overtones.
The history of gas regulation under federal law and how recent changes may affect the electric utility industry, prepared by the State of Indiana.
Energysearch.com is a search engine from the Electric Power Research Institute.