Fact checking sources with good reputations include:
Snopes.com - The Urban Legends Reference Page. Site authors research rumors and include citations and references.
Politifact.com - A Pulitzer Prize-winning site that researches and rates political statements and claims. Rating scale goes from True to Half-True to Pants on Fire!
FactCheck.org - A Project of the Annenberg Policy Center of the University of Pennsylvania also see SciCheck which specifically fact checks scientific claims
Washington Post Fact Checker - Rates claims from true (A Geppetto Checkmark) to "Whopper" (4 Pinocchios)
AP Fact Check - From the Associated Press
Open Secrets - From the Center for Responsive Politics
Be especially wary of anything claiming to provide the "truth" or "just the facts".
True facts and statistics can be manipulated in a number of ways in order to tell a certain story. Many people have fallen for the DiHydrogen Monoxide petition - requesting that water be banned!