I am moving over from Sprint because I believe I can greatly reduce my monthly phone bill with your pricing structure. Here is what I am presently using to get totally free service over WiFi.
Handset: LG Optimus S
I have rooted the Optimus via this thread on Android Forums. It is MUCH easier now. Rooting is not required, just recommended.
Google Voice (free)
::It will require a free account at http://www.voice.google.com. This is the app I use to send all my texts. I've been using it for over a year, and it's stable if not a tad slow.
:: This enables phone calls over WiFi or 3G/4G. I have it set to call over WiFi only. All the other settings worked out of the box for the Optimus. I used this on a Evo 4G with Sprint and it required some tweaking.
With that, I haven't used a single text or minute from my plan with the exception of a few tests. Of course, this won't work when you're in the middle of nowhere, but WiFi is so common that I won't have a problem keeping my bill far below what I used to pay.
I am using this on the Optimus until I sell my previous phones. I'll upgrade to something faster eventually. Maybe. (The battery life is amazing)
A few recommended apps:
SetCPU (1.99, requires root)
::I use this to keep things running smoothly as GrooveIP and and Google Voice are running in the background.
DroidSail (free, requires root)
::The Optimus is a good phone, but with only 512mb of internal storage it's a huge relief to have an app which can move ANY other app onto the sd card. This includes system apps and pre-installed apps.
Quick Settings (free)
::Easy way to turn mobile data on/off and manage WiFi networks on the go.
Eventually I'll try out Locale which will manage my phone settings based upon location.
All in all, Ting will cost me $25 per month at my regular usage without any other phones on the account. The best plan I could get elsewhere would run about $50 per month. By using my WiFi I plan to reduce that to $15 per month. Either way, win/win.