Todoyu is an open source task/project management, time tracking & collaboration application that is built with PHP. The application has a modern, Ajaxed interface where both users & clients can have roles on projects.

The Ajax Availability Calendar is a php script designed to show availability of whatever you might need it for (eg. holiday cottage, hotel rooms etc.). 
Free to use on your website or application and is currently being used (v.2) by hundreds, if not thousands, of websites around the world.