THOMAS is the Library of Congress's website that provides a wealth of information on legislation, including laws, treaties, and other legislative matters.
The Congressional Research Service, which prepares reports on virtually any topic for members of Congress maintains an open website to provide these reports to the public. https://crsreports.congress.gov/
The United States Senate's website can access information on U.S. laws, pending bills, and other Senate business. Its Virtual Reference Desk is particularly helpful in accessing information organized around a particular topic. http://www.usa.gov/
The Legal Information Institute (LII) at Cornell University is one of the earliest public access legal databases formed with the intention to make the law accessible to all in an understandble way. It contains links to federal, state, and other legal resources. http://www.law.cornell.edu/