Skype for Slack - Move your slack discussions to a group skype call
ChatWhale - Gamify your customer communication and loyalty rewards
Calls on Slack - Make voice calls in Slack
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.
CallKite - Your website’s video call center in just one click.
Promoter - The Most Effective Way To Engage And Learn From Customers