Webmin - Webmin is a web-based interface for system administration for Unix.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
ServerPilot.io - Centralized hosting control panel for Wordpress and PHP web sites
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
cPanel - With its first-class support and rich feature set, cPanel & WHM has been the web hosting industry's most reliable, intuitive control panel since 1997.
mailcow - An open source mailserver suite.