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