Mattermost - Mattermost is an open source alternative to Slack.
Slack - A messaging app for teams who see through the Earth!
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Flock - A faster way for your team to communicate
Microsoft Teams - Microsoft Teams provides the enterprise-level security, compliance and management features you expect from Office 365, including broad support for compliance standards, and eDiscovery and legal hold for channels, chats, and files.
RingCentral Video - Live life unlimited. Free video meetings and team messaging in one app that works the way you do.