A few weeks back, Google began rolling out a Gmail Labs feature that enabled people to send text messages to cell phones within the US, but had to pull it back when a showstopper bug related to message transmission was found. Google hopes the second time will be a charm, though, and has finally brought back the SMS feature for their web-based email service.
To use it, one must first enable it through Gmail Labs and then simply input the contacts 10 digit US cell phone number into the chat box on the left. Those who receive SMS messages from the service will get a unique return phone number from the 406 area code to which they can respond. According to Google, messages from the same person will always come from the same number, so you can even bookmark it in your phone and be sure messages will be delivered to the right person.
Of course, many cell phone users in the US get charged on a per-message-received basis, so if you want to avoid hefty bills just reply block or stop to either block a specific contact from texting you or stop the service altogether.