Ringplus.net is having a new years promotion until Tuesday. They have different free plans, but this one is the best. You have to top up $20 when signing up (tax & fees is another 25% in NY, so around $25 total). You can bring any Sprint phone (I'm not sure about prepaid) except push to talk, BlackBerry, and palm. Use the ESN checker to verify. Overages is 2¢ for either text minutes or data. MMS is 4¢ but if you use any messaging app that won't be a problem. And I think on iPhone MMS goes through the internet so it's not counted as one. If your balance reaches zero then they charge another $20. They also require you to post on twitter or Facebook about them when signing up.
The reason why they are so cheap is because when you make a phone call it doesn't ring it plays ads, and when there aren't any ads it plays music/news. ( I set it for classical music during the year and news during sefira.)
If you want to try it out first you can sign up to the "truly free" plan which doesn't even ask for a credit card. But once you sign up to that plan you won't be able to switch to a different free plan because it's only for new members (it could be that the pepper plan is for current members also, check discourse.ringplus.net). So if you have two phones that would be helpful (i think if you cancel a phone you can't activate that phone again for 90 days). During the promo they are also having free port ins (check the discourse for all the details)
The service is regular not slow or bad call quality. The only problem with them is the support which is a little slow but I've never really needed them.
There are options to shut off data text and mms, and other options also in your account dashboard which is useful.
I don't know how long these free plans will go for but I've had them for 6 months already.
And the official rule is that you can only have one phone per person. So please don't abuse the system I would like to keep my free cell phone service going.
Sorry for the long post.