Monket Open Source Ajax Calendar

Monket Calendar is an Ajax enabled online calendar coded in HTML, CSS, Javascript, and PHP. The interface has been designed to make it as easy as possible to add/change/remove events and thereby reduce the frustration of using the browser for entering and managing data. For example, drag and drop events to change dates, drag the start/end of an event to create multi-day events, create and edit events without refreshing the page, all with an iCal style interface.

Requirements: PHP 4+, Apache with RewriteRules enabled support Firefox, Opera, Safari, and Internet Explorer (IE support is still buggy)
Demo: http://www.monket.net/cal/
License: License Free