Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Cronofy - Unified Calendar API for developers
emble - Plan better events with your friends
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Raft Calendar - A fun and beautiful calendar for couples and close 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.