Other
Mayan Calendar
While brief, this simple explanation of the Mayan calendar may clarify for some how its two cycles work together.
The History Cat
The History Cat: Mayan Achievements
Discusses the Maya system of astronomy and their calendar.
University of St. Andrews (UK)
University of St. Andrews: Mayan Mathematics
After a historical introduction, this site from the University of St. Andrews provides information about the mathematics of the Maya Civilization and shows just how advanced they were.
Other
Jaguar Sun: The Maya
A large amateur collection of pages on all the major areas of Maya life and culture.