Open Secrets! Non-Partisan organization compiling research money & politics in the US.
Federal Election Commission (FEC). Collects Campaign finance data for US elections.
Sunlight Foundation Non-Partisan Organization compiles research and fact checking of US politics.
CQ MoneyLine Includes databases you can use to search for the financial details about campaigns, candidates, donors, political action committees, parties, and foreign agents.
Lobbyview Lobby view is a database provided by MIT which provides an abundance of information on political lobbying.