I suggest Virgin Mobile since they allow you to keep the minutes you purchase for 90 days without having to "top-up". When you have to top-up, it is just a min. ammount of $20 bucks, so you can get service for like $80 a year if you don't talk that much. The minutes cost .25 cents for the first 10 minutes, then .10 cents afterwards.
They use sprint's PCS lines for the cell phone, so I haven't had a problem with them.