Cronofy - Unified Calendar API for developers
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
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