[Editor's Note: This API is no longer available. For details, click here.]
From their site: GovTrack.us tracks the U.S. Congress and is a reference for legislation, votes, Members of Congress, congressional districts, and the congressional record. The database that powers the site is also made freely available, and several APIs are available. APIs are URLs on GovTrack that serve XML rather than HTML. They are intended to be called by applications to retrieve data dynamically.