Annenberg Political Factcheck - a project of the Annenberg Public Policy Center. 'Monitors the factual accuracy of what is said by major U.S. political players.'
From the St. Petersburg Times and Congressional Quarterly. Has a 'Truth-O-Meter' scorecard checking the attacks on the candidates (includes explanations). Also see their Punditfact page.
"Dedicated to checking the accuracy of claims by pundits, columnists, bloggers, political analysts, the hosts and guests of talk shows, and other members of the media."