Receiving SMS from Gmail

Gmail users can send SMS messages to their contacts through Gmail Chat. To write back, reply to the message as you would any other text message, and your message appears as a Chat message in your friend's Gmail account.

How can I start sending SMS messages from Gmail?

Anyone with a Gmail account can send SMS messages to their contacts in certain countries through Gmail Chat. To start texting, turn on the Text Messaging (SMS) in Chat labs feature in the Labs tab of your Gmail account settings. Once the lab is enabled, you can send text messages to your friends' phones.
If you don't have a Gmail account, you can create an account at http://mail.google.com/mail/signup.