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.
Radicale - The Radicale Project is a complete CalDAV (calendar) and CardDAV (contact) server solution.
Doodle - Make meetings happen. With Doodle, scheduling becomes quick and easy.