Formerly THOMAS, Congress.gov was created by the Library of Congress in 1995 to make legislative information freely available. It is a great source for legislation enacted since 1973. You can use Congress.gov to find documents related to a particular act, as well as legislative history summaries.
The Government Printing Office (GPO) website provides many valuable legislative links.
Congress.gov and GPO are the most comprehensive websites for congressional information, but both the House and Senate maintain their own websites, which are also good sources of legislative information.