Google has a web page that lets you send free SMS in the US: Google Send to Phone.
This page is used in Google Toolbar for IE and Google Send to Phone for Firefox to send short text messages of web page content.
This feature has been available since last year and it's a simple way to send free SMS, without using sites that require registration.
To find out more about this, including some of the privacy issues, read the FAQ.
Update: In July 2008, the service has been retired and it's no longer available in Google Toolbar or as a separate extension. There are many other services that let you send free SMS, including Gizmo SMS, Text4Free and TxtDrop.