Sending text message from yahoo messenger is what we have been doing from a long time but only when its installed on a computer. But now with Messenger integrated withing yahoo inbox you can send text message to anybody by just pressing a T.
It opens a new text message box which is similar to your new compose mail and it looks liek chat box. Now you can just input the number and send the message. The text message service asks for your country name when you first try to use it.
I tested this with my own number and got it in few seconds. The snapshot contains the text Message I recieved and read it on my computer using the Nokia Pc Suite.
Though the messaging from inbox is free but thereis a limit that the person should reply you back from his mobile device. So if you are looking for free services check out some best services to send free sms or you can also create Google SMS group to send out sms notification to all mobile groups