Dotgo (www.dotgo.com) is an SMS service that allows you to interact with your domain for free through the use of the short code for your top level domain (DOTCOM, DOTEDU, etc). By entering your domain as the first line of your text, dotgo routes the remainder of the request to that domain, looking for an index.cmrl file.
This module puts a listener at the index.cmrl URL and through the use of hooks allows you to define "engine modules" which will process the incoming text (think big - create nodes, retrieve content, whatever!). It also provides an API to respond with text messages and set session variables. It also includes a dotgo_auth module to allow users to link their mobile device with their Drupal user account (for the creation of nodes, etc).
And, DOTGO is free to use! You can even make money by serving advertisements with your content.
See http://dotgo.com for more information.
Patches, architectural changes, documentation, feature requests... all welcome!