I have a second number from Anveo.com that I can forward to any number in the world. They have voice mail and smart call flow routing so you can do things like forward unanswered calls to another number, set up a mini IVR system (i.e. "Press '1' to call me on my mobile", "Press '2' to call my mom", etc.). They do SMS forwarding for some countries, as well as web based SMS.

They also have a nice service that lets you purchase a number in a foreign country and forward it to your own number. My relatives live overseas so I bought a local number in their country that they can use to call me for free. it only costs me a few dollars/month.

None of these features are unique to Anveo, but I've been using them for a few years and have been quite happy with the service.

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.