When I was in Mexico I used Skype to call my family at home (landline). Some years back I bought 10Euro worth of SkypeOut credit, and I've still got lots left.

The calls worked fine from Mexico, but wi-fi connections may be hard to find. There was free wi-fi at my (relatively expensive) hotel, but no such luck at many other (some also expensive) hotels. There's free wi-fi at some airports too, at least after going through check-in. Outside the check-in area the access points appeared to be of the CC-pay type.

Mexico supports GSM phones, but the coverage is spotty outside of towns.

EDIT: Oh, and if you're getting a car, don't drive at night - you'll find the mother of all speed bumps on some of the roads, you literally have to stop the car to go over. You don't want to run into any of those in the dark. And use bottled water for _everything_, don't let any local water touch your lips or toothbrush. Bring some medical whiskey or something, use it to clean your toothbrush in case you accidentally forget yourself (like I did, with dire consequences). The water is probably fine if you're used to the bacterial flora (i.e. if you live there), otherwise don't take any chances.