Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
tripcake - Doodle for travel plans with friends
Cronofy - Unified Calendar API for developers
DUNK - Surprise your friends with secret group plans
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Howbout - Social calendar app to easily make plans & find availability