America Online Instant Messenger (AIM) features sending free SMS to cell phones in the US, supports most of the cell phone providers / careers.
You can add a mobile phone as a buddy in your AIM. You can also send messages adhoc to any US cell phone. Most of the US cell phone careers are supported.
This feature will be extremely useful when you want to send a message to friend in the US or send a notification to someone in the US. You can send the message from anywhere in the world.