You can send SMS text messages to any contact in your Gmail address book right from the Gmail website.
To enable SMS services in your Gmail, go to your Gmail Labs settings page and enable the Text Messaging (SMS) in Chat feature.
To send an SMS from Gmail, first enter a contact’s name in the search box of the Gmail chat window and select Send SMS. Then enter their phone number in the “Send SMS messages”, type your message in the chat window and hit Enter to send the SMS.
The text messaging facility in Gmail is currently available in US, Israel, Pakistan and select other countries.
You can send (and receive) text messages via the embedded chat in Gmail website but not from the standalone Google Talk client. Yahoo! Mail, Windows Live Messenger and AOL AIM also offer the SMS messaging facility.