Schedule Share - Create and share read-only calendars
Cronofy - Unified Calendar API for developers
Ddays - Plan events with friends without the back and forth messages
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
emble - Plan better events with your friends
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.