1. Vodaphone Pay-As-You-Go Cell PhonesI bought the cheapest phone Vodaphone offered over three years ago, and despite heavy use and even some abuse, it's held up wonderfully. My particular model is a small, feature-free Nokia which cost me 50 Euro and came complete with a number of "units" on it - something that is close to minutes, but that's determined by an arcane formula and varies with call distance. I'm a light cell phone user, but to cover six or seven months travel in Greece, I added about 100 Euro and still came home with minutes. All of the Vodaphone personnel I've dealt with spoke English and were very willing to help
Downside: you'll have to find a Vodaphone office. All cities and most towns of any size will have one.
3. Mobal Cell and Satellite PhonesI don't like the per-minute rates on this cell phone system, but the big advantage is that usage is charged to your credit card, so you don't have as much worry about running out of time at a bad moment or far from where you can put more minutes on your phone. If you're going into remote areas, or traveling by sea on a yacht cruise or something similar, they also offer satellite phones for rent at a low daily rate.
Cell Phone Drawbacks: the number they provide you is for Britain, not Greece, so people calling you will have increased charges. And they also charge you for incoming calls.
Nitpicking: Their website immediately launches a distracting audio presentation