Dudle - Dudle is an online scheduling application, which is free and open source.
Teamup Calendar - Cloud-based calendaring using capability URL, application developed in Zurich, Switzerland.
FullCalendar - FullCalendar is an open-source JavaScript event calendar.
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Framadate - Make your polls
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.