Skype for Slack - Move your slack discussions to a group skype call

Zoom - Equip your team with tools designed to collaborate, connect, and engage with teammates and customers, no matter where you’re located, all in one platform.

Calls on Slack - Make voice calls in Slack

Whereby (formerly appear.in) - Collaborate from anywhere - video meetings with no downloads

Pingly - Your communication hub — Email, text & voice in one app

Mornin - An anonymous audio conferencing service