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.
Teamup Calendar - Cloud-based calendaring using capability URL, application developed in Zurich, Switzerland.
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.