One that I didn't know about was 'Fast Follow' on twitter:
Twitter: Whether it is class homework updates, project deadlines, school news or dinnertime conversation starters, teachers are taking advantage of Twitter’s free tool and keeping parents up to date and involved in classroom happenings. In a day-in-age where most parents have cell phones, Fast Follow by Twitter becomes a simple and reliable alert system. To use it, all you need to do is setup a Twitter account and ask parents (and older students) to text Twitter’s shortcode of 40404 with the message “follow [your Twitter username].” From there, parents will start receiving all of your updates via text message in real-time.
Read more here: http://www.weareteachers.com/community/blogs/weareteachersblog/blog-wat/2013/05/30/5-apps-that-engage-parents-in-the-classroom