WebRTC Desktop Sharing is a cool extension. Its functionality will be familar to those who have ever used Hangouts or any screencasting app on a Chromebook.
What this extension allows you to do is share a tab or screen and privately share it with up to 250 individuals. You can, of course, record it like Hangouts does, but you can just as easily have it be a non-recorded, private, live session.
I can think of a lot of scenarios where this technology would be useful. For example, libraries supporting patrons for 1 on 1 support in navigating services, tech support for a small group of teachers, or taking students through an assignment after school hours.
This extension is so simple to use, share screen, copy private url, email or text it, and user on other end pastes it in omnibox.