Well there is always phpiCalendar.
http://phpicalendar.sourceforge.net/nuke/
It is somewhat easy to install however it will not act as a live calendar, it only phases iCal files in to HTML
- Put PHP iCalendar in a web directory or FTP
- Edit the file "config.inc.php"
- Put your iCal files into the directory you specify in config.inc.php
- View calendars in your web browser
Edit: However there is a fork that integrates it with the post- or phpnuke CMS system so the adding to function would be completed on that end.