Appointment Booking Calendar - With the Appointment Booking Calendar you can accept online bookings from a set of available time-slots in the calendar.

Celcat Timetabler - Celcat Timetabler is the complete, state of the art solution for optimising resource utilisation, timetable publication, ad-hoc room booking, attendance tracking, pay claim management...

MakePlans - Online appointment booking - MakePlans

Kairos Calendar - a fully open-source FOREVER continuation of PHPScheduleIt/BookedScheduler All the features without the SaaS takeover.

Dudle - Dudle is an online scheduling application, which is free and open source.

ADE - Resource management and constrained activity scheduling.