A Guide to United States Government and Politics: Lobbying

GovTrack.us, is a tool created by Civic Impulse, LLC. The site helps ordinary citizens find and track bills in the U.S. Congress and understand their representatives’ legislative record, includes a fair amount of historical data, back to 1789, on legislation, legislators, and vote counts.
See the notes on Information Coverage and Timeliness on the site's About page.

Campaign contributions at the federal and state levels, by candidate and donor. Also includes information about PACs, lobbyists, and Congressional committees.

From the Sunlight Foundation, this site offers current information on Bills, Senators, Representatives, Votes, Issues, Committees, and The Money Trail.

MapLight combines three data sets:
Contributions: Campaign contributions given to each member of Congress. (Source: Center for Responsive Politics)
Votes: How each member of Congress voted on every bill. (Source: GovTrack.us)
Support/opposition: Which interest groups and companies support and oppose key bills. (Source: MapLight research team)