The most important things to know about the Mayan calendar are:
1) It is ending in December 2012.
2) The Mayans thought of their calendar as a spiritual gestation process.
A 26,000 year period, separated into 5 different periods - each  5,200 years long.
3) Catholic priest burned virtually all Mayan texts ~500 years ago making understanding difficult.
(Hmm...intervention to save the remaining few?)

History & info - the Mayan calendar - learn about different calendars

Mayan Calendar (Wikipedia)  /  Spring_Equinox / Maya World Studies Center - good calendar info

Campaign for the New Time
(Good graphic - wave harmonic history 13 baktun cycle)

Documentary

National Geographic:
Dawn of the Maya

Available only at nationalgeogaphic.com

Watched Jan 2007

This DVD covers recent explorations of early Maya civilization in Northern Guatemala. Structures of the Preclassic era
from 1000 BC to 200 AD are investigated.
The video has great aerial panoramas of the area.
The findings and translation of wall carvings is interesting.
Not a "must-see," but worthwhile if you have easy access to it.

El Mirador (Wikipedia)

Documentary

History Channel

Decoding The Past - Mayan Doomsday Prophecy

Watch this video below!

Good show that talk about the Mayan calendar and
what the end of the cycle may mean.

Most memorable explanation:
Katun 2 - For half there will be food. For the other half - misfortune.  End of the word of God. Uniting for a cause. 

I've even provided a negative review by one of the
Mayan experts interviewed.

How Not to Make a 2012 Documentary
(By John Major Jenkins)

Maya Rise & Fall - National Geographic Magazine