A plugin to send SMS messages from other plugins. As easy to use as wp_mail().
An easy to use PHP class to send SMS messages. Does nothing on its own, but an implementation of it can be seen at my University's library, where they've used both Scriblio and wpSMS to allow visitors to text information about a book to their cellies.
The plugin is aimed at developers, so the best documentation is in the code, though there is a quick introduction and some screenshts of it in action available here.