SOGo - SOGo is groupware server with a focus on scalability and open standards.
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.
Baïkal - Cal and CardDAV server
Acuity Scheduling - Automate your client bookings, cancellations, reminders & payments using the worlds friendliest online scheduling software.
Xandikos - Lightweight CalDAV/CardDAV server on top of Git.
Reservio - Reservio is a simple online booking and appointment scheduling application.