Google Calendar - Spend less time managing your day & more time enjoying it
Cronofy - Unified Calendar API for developers
Mixmax Calendar - The fastest way to schedule meetings with anyone, anywhere
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Sunsama - Calendar and scheduling for teams
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.