Google just introduced a new feature that allows Gmail users to send free SMS text messages to their contact’s cell phones. These short messages can be sent while using the Gmail chat feature which is part of the Gmail interface. Unfortunately these messages can only be sent to US phones and it is send only — you won’t receive replies.
In order to use this feature you must first enable it via the settings tab in Gmail. Since this is a Labs feature – you should consider it experimental.
At this time the capabilities have not been added to any of the Google widgets or instant message clients, though I would expect that they’d appear soon.