A professional networking website where you can look up the authors of articles and books to see if they're credible.
A product of the Annenberg Public Policy Center, this site is terrific for checking up on political claims.
The Pulitzer Prize-winning Politifact researches the claims of politicians and checks their accuracy.
One of the oldest debunking sites on the Internet, Snopes.com focuses on urban legends, news stories and memes. the also cite their sources at the end of each debunking.
Similar to Snopes but tighter in scope, Hoax-Slayer focuses on email hoaxes, identity theft scams and spam.
While focused primarily on political facts, it covers specific claims in-depth and with plenty of cross-referencing.
Double check image sources and see where they came from.