The Jehovah's Witnesses have predicted it several times within the last 200 years alone
They haven't even existed for 200 years. Their early incarnation predicted the world would end in 1914 and they were thoroughly disappointed when it turned out to be a minor scuffle between a handful of countries.
They've since changed the prediction of 1914 being the end of the world to being the moment that Satan was finally cast out of heaven and down to the earth, hence the massive jump in military technology in WW1 from previous wars, which was apparently because of the devil's influence on weapons manufacturers. They do believe Armageddon is coming soon though.
At one point they believed it would happen within the lifetime of anyone alive during the first world war, but have since changed it to within the lifetime of anyone born within the lifetime of anyone alive during the war.
Nothing will happen, at all. Though I do expect an end of the world at some point with the return of Christ, but who knows when that'll happen. The December 21'st thing, I don't know how anybody came to the conclusion that the Mayan Calendar running out somehow meant anything but whatever.