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.
Cronofy - Unified Calendar API for developers
Acuity Scheduling - Automate your client bookings, cancellations, reminders & payments using the worlds friendliest online scheduling software.
ApPHP Calendar - ApPHP Calendar is a PHP calendar script that may be easily integrated and used with various PHP projects, such as schedulers, event, etc.
Appoint.ly - Appoint.ly helps to schedule meetings quickly and easily through the integration with calendars online.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.