Plan - Plan is a better way to organize.
Cronofy - Unified Calendar API for developers
Outplanr - Turn your to-do list into a real work plan
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Clean Google Calendar - Makes Google Calendar look nicer & launches as a desktop app
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.