[ElKoji]

if you live in the UK or Spain (where Google voice doesn't exist) O2 / Telefonica does something very similar, you can use their service TuGO and you can keep your phone number, receive and send SMS and calls all through internet and you don't even need your sim, or a phone, you can use it on a Browser or an app for Android and iOS (iPad included), on any network, wifi or some disposible sim. The advantage over GV (apart from not being US only) is that whilst you can use it without a sim, you do get the sim for those services that refuse Voip, you get the best of both world. Also TuGO is given at no extra cost, as long as pay monthly you have it.