Apple Calendar Server
http://mark.mckillen.com/article.php?story=20090220074155616
http://www.deanspot.org/content/installing-apples-calendarserver-ubuntu
https://wiki.edubuntu.org/CalendarServer
DAViCal
The project homepage
DAViCal on Ubuntu Users (German)
I got DAViCal to work with Ubuntu but its not the most stable piece of software that I have seen yet.
sudo aptitude install davical davical-doc
Allow network connections on port 11371:
sudo ufw allow 11371
Create /etc/davical/<IP_address or canonical_name>-conf.php
and /etc/apache2/conf.d/davical.conf