FreeBu - Smart free/busy calendar sharing and meeting organizer.
Time Slot Booking Calendar PHP - Time Slots Booking Calendar PHP for online appointment booking and class scheduling.
Letsfind - Letsfind is a fast, easy-to-use, ad-free way to create online (meeting) polls.
Pulse 24/7 - Booking & scheduling mobile app for service providers
FullCalendar - FullCalendar is an open-source JavaScript event calendar.
Dudle - Dudle is an online scheduling application, which is free and open source.