It's hard to beat free, and with FreedomPop, you get it all for free: 200 voice minutes, 500 text messages, and 500MB of data per month. If you want more, you can sign up for an inexpensive plan starting at $5 per month.
The service uses the Sprint network, so you'd want to make sure they have coverage in your area. FreedomPop has been around a while, but according to a detailed report on Re/code, has only now announced they will begin offering service for the iPhone. You can buy a refurbished iPhone 5 from FreedomPop for $349 to use with their service, or you can bring over a Sprint iPhone 4 or 4s. Their voice service uses VoIP, meaning that your call is placed via the cellular data network. And that means you'll get the best quality if you use an LTE iPhone 5 with their service.