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
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
ApPHP Calendar - ApPHP Calendar is a PHP calendar script that may be easily integrated and used with various PHP projects, such as schedulers, event, etc.