Dudle - Dudle is an online scheduling application, which is free and open source.
Cronofy - Unified Calendar API for developers
FullCalendar - FullCalendar is an open-source JavaScript event calendar.
ApPHP Calendar - ApPHP Calendar is a PHP calendar script that may be easily integrated and used with various PHP projects, such as schedulers, event, etc.
Framadate - Make your polls
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.