Federal Elections Comission (FEC)
"administering and enforcing federal campaign finance laws"

OpenSecrets [Center for Responsive Politics]
The Center for Responsive Politics 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.

CampaignMoney.com
CampaignMoney.com is a unique website that lets you see for yourself the hidden world of American political campaigns."

Follow the Money
"The nonpartisan, nonprofit National Institute on Money in State Politics promotes an accountable democracy by compiling comprehensive campaign-donor, lobbyist, and other information from government disclosure agencies nationwide and making it freely available at FollowTheMoney.org."

Silent Partners [The Center for Public Integrity]
This was a study of the money spent by political advocacy groups during the 2000-2006 election cycles. While historical, it provides an interesting window into the influence this money has on the election cycle. This is money that is not controlled under federal campaign finance law.