This year sort of served as good, ironic reminder as to just how ridiculous any Mayan centered doomsday prediction are for December 2012, since 2012 is also a leap year! :-)
You know how we use a base-10 counting system? (10, 100, 1000, 10,000 etc.) The Mayans used a modified base-20 counting system for keeping track of days - the second cycle went up to 18 rather than 20. So they tracked days in cycles of 20, 360, 7200, 144000, 2880000, etc.
We're coming to the end of one of the 144,000 day cycles. Today (March 6, 2012) can be expressed at 18.104.22.168.10 on the Mayan long-count calendar. This calendar will reach 22.214.171.124.19, on December 20, and then turn to 126.96.36.199.0 on December 21. This happens roughly every 400 years. No big deal.