I've been using Virgin Mobile for the last two years. It's not the greatest service, but I had a legit smart phone for $27 a month. I probably wouldn't recommend them if you want to consume a lot of media on your phone, but if you're looking for something that has good navigation and decent web use, it works. I believe they use the Sprint network though.
I'm the type of person that would rather have a mediocre phone experience and save a lot of money.