Cronofy - Unified Calendar API for developers
Pick - The simplest way to schedule a time to meet
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Whatspot - Simple booking system for spaces in companies. Use app to reservations of meeting rooms, car fleet.
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Gameon Active - A booking management platform that helps schools, leisure centers, community halls, and sports facilities of all sizes take their facilities online and start accepting bookings in minutes