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On Bragster, users post dares — for example, "I dare everyone to drink five raw eggs" or "I dare everyone to make as many spins in an office chair as possible (in 1 min)" — and other users respond by posting videos of themselves fulfilling the task. Selected winners get bragging rights points and compete in a site-wide leaderboard. Each initial dare also gets voted up or down, so you can weed through the news for the most interesting dares.
There are even levels to be acquired and Xbox Live achievement or Foursquare badge-like medals for top braggers.
The site has been around since 2006. It's a good match for Guinness, which already allows record-seekers to upload their records to its own website.