Any mac software which allows free international text (otherwise New Zealand,Australia,U.S,Europe,Canada,India,China) which is
Snow Leopard compatible as well

I have seen couple of sites which allows free sms/text (up2 150 characters) in the internet,but haven't find any software which uses this technology.
Few sites offer this service with the catch tht both sender and receiver needs to be registered with the internet service provider

Even a lot free voip call services are available but no sms services are available ? why cant they use voip technology for sms ?