Meetio Room - Meeting Room Booking System
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.
Resource Central - Meeting room & resource scheduling software for Outlook
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Smartway2 - Book meeting rooms, desks & more via mobile/Outlook/desktop
mailcow - An open source mailserver suite.