Rocket.Chat - Rocket.Chat is a Web Chat Server, developed in JavaScript, using the Meteor fullstack framework.
Padlet - Visual boards for organizing anything.
Slack - A messaging app for teams who see through the Earth!
BookWidgets - BookWidgets is an excellent application that let you create engaging exercises in minutes and easily share a varied variety of beautiful and fun activities with your classroom and others.
Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.