Thomas (full-text of bills from 1989 - present; summaries only from 1973 to 1989)
Federal legislative histories are used to aid in determining the intent of Congress at the time a statute was planned and enacted or amended. Courts look to legislative intent when a statute’s meaning is ambiguous on its face.
This tutorial contains a useful introduction to performing legislative history research: Legislative History Research Tutorial (Georgetown University)
Also, be sure to consult Jennifer Morgan's guide on Federal legislative histories: