The Serval Mesh - It works by using your phone's Wi-Fi to not communicate with other phones on the same network.
Padlet - Visual boards for organizing anything.
Briar - Secure messaging, anywhere
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.
WorldChat - Meet new people around the World on WorldChat.tv, the Original World Chat since 2011.
Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.