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.
Calendly - Say goodbye to phone and email tag for finding the perfect meeting time with Calendly. It's 100% free, super easy to use and you'll love our customer service.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Acuity Scheduling - Automate your client bookings, cancellations, reminders & payments using the worlds friendliest online scheduling software.
mailcow - An open source mailserver suite.
YouCanBook.me - YouCanBook.me is a cloud-based service which allows your customers to schedule appointments and make bookings.