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.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Acuity Scheduling - Automate your client bookings, cancellations, reminders & payments using the worlds friendliest online scheduling software.
Cronofy - Unified Calendar API for developers
Doodle - Make meetings happen. With Doodle, scheduling becomes quick and easy.
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.