iCal is the Mac OS X calendar application. One of its features is the ability to publish your calendar to a remote web server via DAV. Other users can then subscribe to the calendar to see updates. Because the calendar stores data in a standard format it is possible for users of other calendaring software to subscribe too. I've successfully connected Mozilla Sunbird to my exported calendar.
This brief tutorial shows you how to set up apache with mod_dav to publish your calendar.