Snopes is widely respected and is one of the oldest fact-checking sites on the internet.
OpenSecrets.org is an independent, nonpartisan, nonprofit research group tracking the influence of money in U.S. politics.
Politifact is an independent, Pulitzer Prize-winning site created by the Tampa Bay Times.
FactCheck.org is a nonpartisan, nonprofit organization that focuses on U.S. politics.
AllSides offers the same news stories from multiple viewpoints, e.g., left, right, center.