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.
Meetingbird for Gmail - View your calendar and schedule meetings right from Gmail
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Calendly for Chrome - Schedule meetings without leaving your inbox