Snopes is the "definitive fact-checking site" and urban myth site. Editors painstakingly track down and verify sources of information to confirm or dispell the internet rumors people send in. It also posts news-- so use the Fact Check section to find their methodical analyses.
When reading Snopes, note the sources that are referenced, including people and publications, and the analysis done on the information received.

This project of the Annenberg Public Policy Center posts articles with detailed fact-checking public policy, health, science, and Facebook/internet/online rumors. In particular, take a look at the video on Internet rumors under Viral Spiral (http://www.factcheck.org/hot-topics/ )

"PolitiFact is a fact-checking website that rates the accuracy of claims by elected officials and others who speak up in American politics." Staff are from the Tampa Bay Times, owned by the Poynter Institute. Politifact's most famous feature is the Truth-o-meter. Pay careful attention to the analysis of information the Politifact journalists use-- this is often a source of disagreement with them.

This list, from the International Society for Technology in Education, includes other fact-checking websites.