Just noticed this on Gmail

Doesn't work with AIS 1-2 call though, only DTAC and True



Send a free SMS to your friends from Gmail

Did you know that you can now send free text messages (SMS) to your friends directly from your Gmail account?
Try Gmail SMS today and start texting your friends from your computer.



Note: Sending and receiving a message from Gmail to any phone is free. When replying with an SMS from phone to Gmail, you will be charged a regular SMS rate by your mobile provider. Gmail SMS is currently available for dtac and True Move subscribers.



http://www.google.com/mail/help/intl/en_th/sms.html