The Presidential candidates have their own sites of course. Check out these two sites that claim to keep the politicians honest by checking the facts.
FactCheck is a good one. The site is from the University of Pennsylvania's Annenberg Public Policy Center. FactCheck accepts no funding from business corporations, labor unions, political parties, lobbying organizations or individuals. It is funded primarily by the Annenberg Foundation.
Politifact is from the St. Petersburg Times and Congressional Quarterly, a trusted name in the information world. I like their Truth-O-Meter. Pictures make it so much easier.