Cronofy - Unified Calendar API for developers
eezy - Daily adventures for ambitious people
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
tripcake - Doodle for travel plans with 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.
Howbout - Social calendar app to easily make plans & find availability