nivcaner

Message Mate is a simple but wonderful slider add-on to your website that advertises a phone number which current and prospective customers can text with one click to reach you or your employees.
Your customers would rather text than call you. In fact, 76% of young adults prefer to interact with companies via texting (and not through calls, emails, live chat or contact forms). And thankfully for your business, texting is a more efficient and less distracting means of communication.
Texts also have much higher engagement potential with a 97% open rate, at least three times higher than email.
Up to three users can receive and respond to incoming messages via SMS, email, and a web dashboard. And you can even add automated responses and post-conversation rating requests. 
Sell more, with Message Mate. Choose any US area code (subject to availability), and get your first 14 days for free.

Message Mate

Message Mate lets customers text you from their phone or computer. Reply via email, text or dashboard.

Casey Bisson

An easy to use PHP class to send SMS messages. Does nothing on its own, but an implementation of it can be seen at <a href="http://library.plymouth.edu/read/222334" rel="nofollow ugc">my University’s library</a>, where they’ve used both <a href="http://about.scriblio.net/" rel="nofollow ugc">Scriblio</a> and wpSMS to <a href="http://maisonbisson.com/blog/post/12094/scriblio-feature-text-this-to-me/" rel="nofollow ugc">allow visitors to text information about a book to their cellies</a>.
The plugin is aimed at developers, so the best documentation is in the code, though there is <a href="http://maisonbisson.com/projects/wpsms/" rel="nofollow ugc">a quick introduction and some screenshts of it in action available here</a>.

wpSMS

A plugin to send SMS messages from other plugins. As easy to use as wp_mail().

Message Mate vs wpSMS