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