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.
Slack - A messaging app for teams who see through the Earth!
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Workplace by Facebook - Connect everyone in your company and turn ideas into action.
Mattermost - Mattermost is an open source alternative to Slack.
Startup Security Program by Templarbit - Security practices & tools required to close large deals