I was on Airvoice for a couple of years, but the group texts to non-iPhones became problematic (Airvoice uses ATT towers). I switched to Red Pocket CDMA (Verizon towers) and that works. I stick with their $10 plan, but they do have a $19 plan with unlimited/unlimited/1GB (which seems like a lot of data to me).
Red Pocket also offers T-Mo and Sprint at the same prices; it just depends on what you want.
Their ebay store offers yearly plans at a discount, but I haven't pulled the trigger on a whole year at once.