I have a VirginMobile phone. It costs $5 per month for more minutes than I use. They bill my credit card $15 every 3 months (it would be $20 if you use the top-up cards from Walmart), and when the balance drifts up too high they sometimes skip a quarter.
The only downside is that the target market is 20 year olds, so the ads and stuff are obnoxious.