[lokl]

I've been very happy with Tello. $5/month if, like me, you barely use your phone. Their 500 MB/month data plan with no voice minutes + one-time $20 pay-as-you-go minutes. Splitting data and voice like this means no taxes or fees, the total monthly bill is $5.

[udkl]

I'm similarly on RedPocket and can recommend it. Good customer support over chat and good plans if you rarely use your phone or data. Their Tmobile MVNO plan goes as low as $30 per year and you can buy it from their store on eBay - 200mins, 100SMS, 500MB if I remember right.

[arantius]

I do this, except Google Voice for calls over data, usually via WiFi.