I've been trying out the free service Intazaar which offers 30 free SMS a day to Australian users. So far I haven't been charged a cent and neither have the people I've sent messages to so it looks like the real deal! Really saves you money on your phone bill or credit. Signup is free and all it seems to do to advertise is send a message to the first person you sent a message to from the service saying “I just sent you a free message from Intazaar.com. This is a once off message” and never does it again. What's great about the service is that the messages come straight from your mobile number so people can just reply to them and they go straight to your mobile.
Sorry, rest of the world, this service is only for Australians.
Intazaar.com