Firstly, the Biblical rapture. Even if we assume for one moment that The Bible is one hundred per cent true, there’s absolutely nothing in it which suggests that the rapture is due to occur on December 21st 2012, no date is given (if anything, The Bible implies that it should have happened years ago). Plus, The Bible says that nobody can know when the end of the world is coming, only God knows, so it would be awfully strange if there were a way to figure it out based on cryptic clues wouldn't it? Since God knows everything, past, present and future, when it says that nobody but God knows, you can assume it means, nobody but God will ever know.
Secondly, the Mayan calendar. December 21st is when the ancient Mayan calendar ends, and some people believe that this indicates the end of the world. Oddly though, the people who believe the calendar points to the apocalypse, are not the people who follow the old Mayan religion. Those who do follow it complain because the Mayan religion does not contain an 'end of the world' event, and they don't like others imposing one on it. Researchers tend to think today, that the calendar ends on that date, because of some mathematical measurements linked to the sun and the Earth.
Thirdly, Nibiru. Apparently a planet is going to crash into the Earth tomorrow (a bit like that time in Doctor Who when Gallifrey came back and nearly crashed into us). But this is rather silly, I mean, with the other two, they feature supernatural aspects and so you wouldn't see signs of them coming, but if a planet was heading towards Earth it'd easily be seen in the sky. Since nobody can see one, I'd say we're pretty safe.
Fourth, alien invasion. Some people believe that the world leaders are secretly shape shifting alien reptilian humanoids… Accepting that alone as truth is awfully hard, but even if you do, it's even harder to see the logic in predicting tomorrow as the day more of them arrive for a full scale alien invasion. If these shape shifters are among us, then they'd know that people have predicted tomorrow as the day of their attack, they'd know people are preparing for some kind of disaster and will be much harder to fight than on another day. Surely, it'd be easier to just come a day early or something? I'll admit, I don't know that much about this theory, but it just seems to have so many holes in it.
Finally, planetary alignment. All the planets will be lined up with the sun tomorrow, so apparently that'll put a large dose of radiation into the atmosphere, or shoot out deadly solar flares, that will wipe life from the face of the Earth. Except that, it already happened once before, and before anybody thinks it, it wasn't when the dinosaurs went extinct, it was when humans were around. So, if it can't even wipe out the tiny human population of thousands of years ago, why should the even bigger population of today be worried? They shouldn't, because nothing happen because of it.
So that's all of the main theories debunked. If anybody was worried by them, I hope I helped a little. And, on the off chance that there’s a big coincidence and tomorrow is the end of the world, I better just add, I love you dearly.