You didn't think Wikipedia was all original content, did you? According to Webempires, the online encyclopedia cites and references over two million different websites. You can find the entire exhaustive list here if you've got a couple of weeks to spare, or check out this handy infographic which highlights just the top 50 sources.
Books.google.com, web.archive.org, and news.bbc.co.uk round out the top three, which makes sense given they're all original sources. But most surprising is the inclusion of both MySpace (which beats out Facebook) and cricketarchive.com in the top 50. Apparently teenage gossip is a vital source of fact-checking, and cricket outranks any soccer-specific sport sites.
Images by Webempires