Other
Mesoweb: The Temple of the Inscriptions Tablets
Wonderful PDF files of various tablets found at the Temple of the Inscriptions in the ancient Maya city-state of Palenque.
Wikimedia
Wikipedia: List of Maya Sites
Pre-Columbian Maya archaeological sites are listed here, with links to Wikipedia pages about each.
Other
Visit Cancun: Mayan Ruins
Visit Cancun is a tourism site that gives some basic information about a variety of Mayan ruins.