http://en.wikipedia.org/wiki/2012
According to the Bible Code the world will end due to a collision with a meteor or comet.
According to spiritualist writer Bob Frissell, John Major Jenkins and others, humankind will ascend into a higher state of collective consciousness.
The Olduvai cliff will begin and permanent blackouts will occur worldwide, according to "The Peak Of World Oil Production And The Road To The Olduvai Gorge" by Richard C. Duncan.
December 21 - End of the great cycle of the Maya calendar's Long Count, and thus the alleged end of our world (the end of the cycle is dated December 23 by some calculations). Also interpreted as a change in human consciousness: the end of the world as we know it but the start of a new one.
Terence McKenna's Novelty Theory claims that time is a fractal wave of increasing novelty that ends abruptly in 2012.