WeTxt is a free, group text messaging service that allows you to send free group text messages from your phone or from the web. It also includes free reply-to-all messages, 2-way text messaging between cell, email, and the web, a mobile calendar with text reminders and text message only games.
This could be very useful for teachers and schools to send messages to students and parents and improve communication, especially for people without smartphones.
ClassPager - send students quizzes and reminders
ClassParrot - easy, safte text messaging for educators
Celly - free, Instant Group Texting and Polls