MAPLight.org, "Money and Politics: Illuminating the Connection" is a public database meant to "illuminate" the connection between campaign donations and legislative votes. According to the website, "Elected officials collect large sums of money to run their campaigns, and they often pay back campaign contributors with special access and favorable laws. This common practice is contrary to the public interest, yet legal. MAPLight.org makes money/vote connections transparent, to help citizens hold their legislators accountable."
MAPLight combines three data sets: Bill texts and legislative voting records, Supporting and opposing interests for each bill, and Campaign contribution data from the Center for Responsive Politics and the National Institute on Money in State Politics in order to make visible key information that could never before be determined easily.
They also have a cool Presidential Money Race widget that you can add to your blog or website.