Availability Booking Calendar PHP - PHP Availability Booking Calendar is open source php booking system for online reservations.
Cronofy - Unified Calendar API for developers
Time Slot Booking Calendar PHP - Time Slots Booking Calendar PHP for online appointment booking and class scheduling.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
MarketPress - MarketPress is the best free eCommerce plugin available for WordPress.
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.