The pyramid is called El Castillo, or temple of Kukulcán, with Kukulcán being the Maya's feathered serpent god. Interestingly enough, the temple you can see today has been built over another temple that is now hidden inside the pyramid - but unfortunately visitors are not allowed to access it anymore.
The picture above shows the restored side of the temple; on the other two sides and the structure on top, restoration work is in progress. This is what it looked like when I visited - quite a difference to the restored sides, isn't it?
Chichén Itzá is the Maya site that receives the most visitors - there are estimates of 1.2 million visitors per day, which breaks down to more than 3.000 per day. Luckily, September being the low season, there certainly were fewer visitors there today. However, the amount of visitors makes it a profitable site for souvenir vendors, and so all the paths in Chichén Itzá are lined with souvenir vendors. While I understand that they are just trying to make a living, I felt that they were a particularly obnoxious bunch: always walking up to people and offering stuff for 'one dollar' or 'only ten pesos', regardless of the real price for their stuff. When people fall for them, they quickly adjust the price to press as much money as possible from the tourists. I got really tired of saying 'no gracias' about a thousand times that day.
The temple of Kukulcán isn't the only building worth seeing in Chichén Itzá, though: the site is rather large and has many other ruins. This, for example, is a richly adorned building in the nunnery complex. Don't let the name fool you - the buildings almost certainly did not serve as a nunnery. They were given the name by the Spanish because the layout of the buildings resembled European nunneries.
Notice that El Castillo and the nunnery buildings have totally different architectural styles. The nunnery was built in the Puuc style, and El Castillo is said to have been influenced by Toltec architecture. Wikipedia doesn't seem to be so sure how the Toltec influences came to be there; the theories range from military conquest in either direction to loose relationships of some kind.
The courtyard of 1000 columns is another part of Chichén Itzá. Back in the day, these probably sustained a big roof. They sure needed lots of columns to create a big room like that! At another building, the Mayan advancement in construction technology was clearly evident: the columns there were much higher, and there were significantly fewer of them.
This next temple is called the ossuary, or temple of the high priest. The serpents decorating the staircase are presumably another reference to Kukulcán.
Chichén Itzá also had a building that is thought to have been an observatory:
And, finally, here's another look at the temple of Kukulcán - it's just too beautiful and perfect to close this post with any other picture ;-)
