The U.S. International Trade Commission (USITC) publishes and maintains the HTS and provides technical information on its structure and modification.
However, the Bureau of Customs and Border Protection (CBP, formerly the U.S. Customs Service) of the Department of Homeland Security is solely authorized to interpret the HTS, to issue legally binding rulings or advice on the tariff classification of imports and their treatment upon entry into the United States, and to administer the customs laws.
Links to tariff schedule resources:
U.S. Harmonized Tariff Schedule (by Chapter)
U.S. Tariff Database (lookup specific product)
U.S. Tariff Programs
Future U.S. Tariff Rates
U.S. Customs Rulings
U.S. Trade Data
U.S. International Trade Commission
Customs and Border Protection
World Customs Organization
National Customs Websites