We've been using Calcium, the web-based shared calendaring software from Brown Bear Software, for some time now. It works very well, especially for a distributed organisation like ours, and the support from the Brown Bear staff is superb.
Until recently, however, I couldn't see my appointments except when connected to the net. Not really a problem - I'm almost always online. But the latest Calcium allows me to access our calendars in iCalendar format, meaning that I can subscribe to them with iCal (or Mozilla Calendar). Now, not only do I have a (read-only) copy of the corporate calendar with me on my laptop, but also, through the magic of iSync, on my iPod. Very sweet! And Calcium can also subscribe to published iCal calendars - it calls them 'Add-Ins' - so you can create content from your Mac.
I have no connection with Brown Bear except as a satisfied customer.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031124191032640