Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.

Duolingo - Duolingo is a free language learning app for iOS, Windows and Android devices. The app makes learning a new language fun by breaking learning into small lessons where you can earn points and move up through the levels. Read more about Duolingo.

Groups.io - Email and web based discussion group software, includes archiving, emails, wiki, polls, file...

Eduflow - Digital learning that actually teaches students something

GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces

Backchannel Chat - Backchannel Chat is best in class discussion tool that is intended for teachers to streamline their class activities.