I have a phone that uses AT&T pay as you go. I got it at either Radio Shack or WalMart 3 or 4 years ago. It's not my main cell phone, I got it to take when I go on hiking trips and it works really well. It's a Nokia. You just buy a $10, $20, etc. card from Radio Shack or an AT&T/Cingular store to refill the minutes. The minutes are good for 45 days and if you refill it before the 45 days are up the minutes rollover. If you don't, you lose whatever you had leftover.
The best things about it are the only up front cost is the phone which was about $50, no contract and no monthly costs, if you lose it or it gets stolen you're just out the cost of the phone, no one can stick you with calling charges. The worse thing about it is that it's really only practical to refill it from the cards using a home phone because it takes 5-10 minutes to go through all the numbers and crap, but it's a toll free call. You can also refill it using AT&T's website, but I just use the cards.