Meetio Room - Meeting Room Booking System
Cronofy - Unified Calendar API for developers
Resource Central - Meeting room & resource scheduling software for Outlook
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.
Smartway2 - Book meeting rooms, desks & more via mobile/Outlook/desktop
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.