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.
Appointment Booking Calendar - With the Appointment Booking Calendar you can accept online bookings from a set of available time-slots in the calendar.
WPAmelia - Award-winning WP booking plugin that manages services, appointments, customers, events, analyze business performance, and accept payments online.
Dudle - Dudle is an online scheduling application, which is free and open source.
Cal.com - Cal.com (formerly Calendso) is the open source Calendly alternative.
MakePlans - Online appointment booking - MakePlans