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

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.

Raft Calendar - A fun and beautiful calendar for couples and close friends

Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.