FactCheck.org is a project of the Annenberg Public Policy Center of the University of Pennsylvania. This Pulitzer prize-winning site fact-checks claims made by the president, members of Congress, presidential candidates, and other members of the political arena. It mainly reviews TV ads, debates, speeches, interviews, and news releases. Viral Spiral is a section of FactCheck.org devoted to internet rumors.
Politifact is a project of the Poynter Institute and has also won a Pulitzer Prize. Politifact fact-checks claims by politicians at the federal, state, and local level, as well as political parties, PACs, and advocacy groups. You can also sign up for weekly emails fact-checking the latest claims.
The Washington Post's Fact Checker assesses claims made by politicians or political advocacy groups.
See a clear example of how foreign interference works from the US Cybersecurity and Infrastructure Security Agency