PhpGroupWare
Website: http://www.phpgroupware.org
phpGroupWare, formerly known as webdistro, is a multi-user groupware suite written in PHP and part of the DotGNU project. It provides about 50 web-based applications including a Calendar, Addressbook, an advanced Projects manager, Todo List, Email, and File manager.
The calendar supports repeating events and includes alarm functions. The email system supports inline graphics and file attachments. The system as a whole supports user preferences, themes, user permissions, multi-language support and user groups. It includes modules to set up and administer the working environment. The groupware suite is based on an advanced Application Programming Interface (API).