Pick - The simplest way to schedule a time to meet
Cronofy - Unified Calendar API for developers
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
ApPHP Calendar - ApPHP Calendar is a PHP calendar script that may be easily integrated and used with various PHP projects, such as schedulers, event, etc.