If you have an old phone from sprint, or don't mind getting a sprint phone from somewhere, you should take a good look at ting.
Http://ting.com
They have no contract plans, it's all based on how much voice, messages, data you use. You don't select a plan, at the end of the month they put you in each rate slot for the three services that will cost you he least and charge you that.
As an mvno, they don't have the latest phones, and they are on sprint, so if you need verizon, or the latest android or apple phone, you won't benefit.
But otherwise you can get a great deal on voice, data, and messages through them. Adding another device only adds $6 more per month, and all the minutes and data go into the same bin so you can take advantage of the discounts at higher usage levels, similar to a family plan.
I value my iPhone 5 (they only support up to the 4s) and verizon, so it's not for me, but if I could switch I'd go from $90/month for my iPhone and iPad down to about $47/month, with the same usage. The fact that I'd get charged for actual usage rather than choosing a plan and paying for overages is huge for me, as I'll go for months with minimal usage, then have a vacation where my usage skyrockets, and right now I have to manage that ahead of time.
I'm not a shill for them, but I might as well be. I think a lot of People here could benefit by taking a look at them.