SyncThemCalendars allows you to synchronize calendars across Google accounts
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.
Dudle - Dudle is an online scheduling application, which is free and open source.
OneCal.io - Sync Outlook and Google Calendars in real-time, share booking links, and view all your calendars in one view.
Appointment Booking Calendar - With the Appointment Booking Calendar you can accept online bookings from a set of available time-slots in the calendar.
Acuity Scheduling - Automate your client bookings, cancellations, reminders & payments using the worlds friendliest online scheduling software.
Pulse 24/7 - Booking & scheduling mobile app for service providers