WordPress Text Message
GPL 3+ license
Add a user signup form for SMS text updates. Users enter cell number and (US based) carrier to become members - website owner sends out updates either manually, for every new post, or for specific page updates.
Plugin has both widget and shortcode support. New 2.03 version enhanced to use standard nonce WP security and captcha in widget for spam prevention. Updates now sent out in batches to support large subscribers lists (with cron queue that works in the background).
Download details and screenshots of plugin options at the official page: