Acuity Scheduling - Automate your client bookings, cancellations, reminders & payments using the worlds friendliest online scheduling software.
SOGo - SOGo is groupware server with a focus on scalability and open standards.
BookSteam - BookSteam Online booking and appointment scheduling software for businesses worldwide. Easy to use, cloud based web scheduler that includes class and group bookings. Calendar management and schedule organization, trusted by thousands.
Baïkal - Cal and CardDAV server
MINDBODY - Find and book fitness and beauty classes near you.
Radicale - The Radicale Project is a complete CalDAV (calendar) and CardDAV (contact) server solution.