Kalkun is an open source web-based SMS (Short Message Service) manager.
It uses Gammu SMSD (part of gammu family) as SMS gateway engine to deliver and retrieve messages from your phone/modem.
Documentation is on the wiki: https://github.com/kalkun-sms/Kalkun/wiki
Development and source code can be found on github: https://github.com/kalkun-sms/Kalkun/
Frosty is a php based application that will allow you to send sms messages to a wide variety of people. Frosty uses email-to-text and google voice and can send a single message up to 100 times to one recipient or to 10000 unknown recipients!
SMSec is a free , open source, GPLed implementation of two-factor authentication via SMS (Short Message Service). It uses SMSD to "talk" to GSM modems, provides a flexible XMLRPC web-service for web-developers integrate SMSec into their web-apps.
mCast is an application that broadcasts short text messages to mobile phones. It transmits a single message to multiple users via both SMS gateways and other wireless service providers' websites (i.e. for providers who do not use GSM networks).