Mayan Calendar: The last Mayan cities were conquered by the Spanish in 1697, so that's pretty much when their world ended. As Diggo said, it's the end of a cycle, just as our main cycle ends every December 31. Do you go to a bookstore, see the calendars, and say, "omg, the world is ending December 31st!"
Solar system crossing the galactic plane: Complete crap. We don't know the exact location, but we're "north" of it and moving away from it, not towards it.
Betelgeuse supernova: Even if it DID go supernova 426 years ago and became visible in 2012, it would have no effect on Earth, but it could just as easily have another million years of life left.
Asteroid Toutatis: Will pass 5 million miles from Earth.
I could go on, but since the Mayan calendar is the entire basis for the date everyone uses, there's no point. There's nothing to debate. We might as well debate the existence of the tooth fairy.