Host a backchannel discussion.
Chatzy - Start a free, private chat, or create a virtual chatroom for more flexibility.
HootCourse - Take class conversation online. Using their accounts from Facebook or Twitter, students and teachers can hold online discussions.
PrimaryPad - Using this tool, students can hold backchannel discussions as well as taking collaborative notes on the lecture.
TitanPad - Another collaborative note-taking tool with a backchannel feature.
Today's Meet - An easy-to-use backchannel site without the need for a Twitter account. Set up a room, decide how long you want the room to be active, then give your audience the room's URL. Free to use with no ads.