this is a service that I've yet to try out, but it's pretty interesting
republic wireless - The Mobile Network that Runs on Freedom
$19/month for unlimited data/call/text provided that you use wifi when you can, when you can't you use sprint towers. Seems like sprint's testing a new connectivity model with this.
it isn't much different than buying a phone and using it with wifi all the time and using a prepaid sim for on the road, but most prepaid sim cost around $10-20/month as well and they have caps