Teamup Calendar - Cloud-based calendaring using capability URL, application developed in Zurich, Switzerland.
Cronofy - Unified Calendar API for developers
Teamweek - A project management software to help you get the best overview. Plan fast, change plans faster!
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.
Google Calendar - Spend less time managing your day & more time enjoying it
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.