There are many different predictions coming from more than just the Mayan calendar.
There will be an alignment of the Earth, Sun, and the center of the Milky Way on 12/21/2012 (Winter Solstice). That has sparked some of the hypotheses about 2012. No one quite knows what this alignment will bring.
Other hypotheses revolve around a possible global awakening in 2012, that will essentially allow humanity to evolve to a new level. Whether this will be a peaceful awakening or a global catastrophe that causes the awakening, I guess we'll find out. I tend to think it will be the latter, though I hope for the former.
Another one involves a shift in the magnetic poles, which has happened multiple times in the past and is due to happen again.
Plus, there are a whole slough of other natural phenomenon that we are ultimately due for: a comet or asteroid hitting us, us going through a highly turbulent area of the galaxy, etc...
All kinds of goodies. There are a ton of books about this.
I have been wondering about the various sources for this also. I had heard from an astrologer friend that there is a general shift going on, represented by shifts in the outer planets, and that 2012 is when certain of these culminate, which doesn't necessarily mean anything will happen right in 2012, but rather that this whole period - a few years before and after - is a big transition period. From her perspective everything going on now politically and economically is part of this shift, but we may not really see the full results of the shift until we look back on it many years after 2012. Have you heard this view?
My big concern with a lot of what I have read about this theory is the fear-mongering that is going on. It is just like 2000, when many people used the idea that the world was going to fall apart that year to frighten people into adopting their views, etc.