Teamup Calendar - Cloud-based calendaring using capability URL, application developed in Zurich, Switzerland.
Cronofy - Unified Calendar API for developers
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.
Kin Calendar - The simple and connected calendar
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.