A Chrome Extension called Send Your Email to SMS from CloudHQ lets you send and receive SMS messages in Gmail.
After installing the Extension and doing a simple setup, you can simply choose a contact from Google Contacts, and send them an "email" from Gmail, which arrives on their phone as an SMS.
If you're an AT&T, you can follow an additional setup to make the SMS appear to come from your phone, and if you've got an iPhone it all appears to operate within Messages.
One benefit of sending from Gmail is that you can send a single message that includes both email and SMS recipients.
You can also tell the extension to send you the email via SMS if it's from a specific person or contains a specific word or phrase in the Subject line.
It sends the entire email. In my experience, it can take a few minutes to get the alert via SMS.
This is going on my List of Chrome Extensions.