BigBlueButton

BigBlueButton is a web conferencing system designed for online teaching and learning. It has real-time audio, video, screen sharing, slides, polls, shared notes, whiteboard, and breakout rooms. Due to the open-source license, anyone can setup their own BigBlueButton server. BigBlueButton can be used as the default virtual classroom for many major learning management systems (LMS) including Moodle Cloud, Sakai, Canvas, Jenzabar, and D2L.

You can find tutorials for teachers here.