Cronofy - Unified Calendar API for developers
Mixmax Calendar - The fastest way to schedule meetings with anyone, anywhere
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Calendly for Chrome - Schedule meetings without leaving your inbox
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Polymail Calendar - Schedule meetings instantly over email