Cronofy - Unified Calendar API for developers
Appointment Booking Calendar - With the Appointment Booking Calendar you can accept online bookings from a set of available time-slots in the calendar.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Dudle - Dudle is an online scheduling application, which is free and open source.
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
MakePlans - Online appointment booking - MakePlans