creative commons licensed (BY) flickr photo by Jason A. Howie: http://flickr.com/photos/jasonahowie/7910370882
“What exactly makes an app “essential” is open to interpretation. For pure productivity, you could consider the direction of Google Drive, Skype, Zoom Notes, iAnnotate–maybe a gradebook app, Class Dojo, etc.
But what if your classroom if is full of open-ended projects and you need to constantly communicate with students, parents, and the community? Google+, Google Hangouts, Remind, DIY, and maybe Trello?