Slack

wikipedia:Slack (2013)

• Internet Relay Chat (IRC)
• Jitsi
• Mattermost (2015)
• Matrix (client: Riot)
• Rocket.Chat (at least 2016)
• XMPP
• Zulip
• Keybase (open-source client with proprietary server)
• Wire (Open Source - End to End Encrypted)

See also

• XMPP, Profanity, Matrix (protocol), MQTT, CoAP, AMQP, MQTT, Libjingle, Mellium
• Mail, email, Notifications, Gmail, Hotmail, ProtonMail, Gmail API, Amazon WorkMail, Zoho mail, Email alias, iCloud, Custom email