With election time looming, there is increased interest in who is supporting who - including donations, PAC money, and data about lobbyists. The text about these recommended private resources is taken from the upcoming 4th edition of The Manual to Online Public Records.
OpenSecrets.org is an extremely useful site. Per their web page "...is the nation's premier research group tracking money in U.S. politics and its effect on elections and public policy. Nonpartisan, independent and nonprofit, the organization aims to create a more educated voter, an involved citizenry and a more transparent and responsive government."
Another recommended site is FollowTheMoney.org which is maintained by the National Institute on Money in State Politics. The Institute provides a searchable database with substantive profiles on candidates and issues, insightful reports and reliable data for all 50 states. This excellent web page also has many, many other database searches available on a subscription basis only.
Also, a great searchable directory of lobbying firms is found at LobbyData.com. Search by name, the agency involved or by issue.
The VoteSmart site tracks many facts about candidates including campaign contributions; however, data may be lacking for local politicians. The site includes voting records and evaluations by special interest groups.