Cronofy - Unified Calendar API for developers
Plan - Plan is a better way to organize.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Outplanr - Turn your to-do list into a real work plan
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