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.
Mixmax Calendar - The fastest way to schedule meetings with anyone, anywhere
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Clean Google Calendar - Makes Google Calendar look nicer & launches as a desktop app