Dudle - Dudle is an online scheduling application, which is free and open source.
Cronofy - Unified Calendar API for developers
Pulse 24/7 - Booking & scheduling mobile app for service providers
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
MakePlans - Online appointment booking - MakePlans
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.