December 21, 2012 is the day that the world ends according to some.
There are all sorts of web sites about it. An example is http://www.december212012.com/.
Even WiseGEEK has a page.
There is, according to some, an astronomical object named Nibiru approaching Earth which will arrive on that date. Among the more interesting claims in my opinion is that the Vatican has built two telescopes on Earth and is launching a satellite for the explicit purpose of tracking Nibiru. There does seem to be some disagreement about Nibiru though including its name.
Apparently though it is Nibiru that is causing the magnetic pole of the earth to shift and that shift will continue to increase with some dramatic even on December 21, 2012. The dramatic event is a collision or near miss. As you might guess not everyone agrees.
There is a Survive 2012 site if anyone is interested in surviving.
There is an article on NASA about a new planet.
That's dated 7.29.2005 and the image is from October 21, 2003.
Thought that was interesting.
A good part of the 2012 stuff comes from Mayan calendar information. Some of it was popularized in the Indiana Jones film about crystal skulls.
Some of the phenomenon predicted include total shift of magnetic pole, big change in weather, big change in oceans, melting ice caps, earthquakes, volcanoes, Sun spots, shortages in food and water, economic disruptions because of shortages, wars because of shortages, and so on.
Oh and God and Jesus will be revealed to have been ancient astronauts and Christianity, Judaism, and Islam have all been covering this up for millennia.
I may have left out some stuff but I suppose you get the idea.