G-Chat on Google.
Basically all you have to do is go into "Settings" at the top right hand side of your G-Mail home page, click on "Labs", then scroll down to near the bottom of the applications and click on "enable". When the homepage reloads, go to the G-chat window, click on your friend's name, and under the "Video & more" option choose "Send SMS".
First of all, the service only works with American cell phone numbers for the time being, so
while you can send the SMS from anywhere in the world, it has to be someone in America
receiving it. Second of all, be aware that the person receiving the text message is still being
charged for them, so unless they have an unlimited texting plan, writing a bunch of responses
like "LOL" or "hm" is going to cost them a small fortune.
And as always, if you want to send an SMS message to the rest of the world for free, for that use free SMS service to do that ..