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
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Kin Calendar - The simple and connected calendar
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.