Originally Posted by Middle_Seat

Not an app, but an entire phone company: RepublicWireless.com

They sell Android phone which have been modified to automatically use wifi as much as possible and cellular signals as little as possible, for phone, text (some issues) and data. You pay for your phone ($249 now, I believe) upfront, then pay ~$22 / month with no contract, no early termination fee. You get unlimited minutes, unlimited texts, unlimited data.

When it's using the cellular network, the phone is on Sprint. So, it's not a GSM phone and has no SIM that can be swapped out. If you're international the cellular system won't work, but if you can grab a wi-fi signal you've got a working phone.

I've been a customer since ~December and have been pleased with the service and even more pleased with the price.