Very slick calendaring product, website and service that publishes and subscribes to iCalendar files. These guys are on the ball and it's written in .NET too. The UI is clean and pretty but I've only just experiemented a little with it so that's just a first impression. I like the way it aggregates multiple calendars.
Unfortunately, it uses a publish/subscribe model which is fine for the Internet but it doesn't allow for shared editable calendars which is what I want. I've put in a feature request and have started looking at it with FileMon.
From reading their docs, I also found they've made an iCalendar .NET Parser available as Open Source. Nice touch.
Of course there is competition. On Windows, other iCalendar software includes WinDates and Mozilla Calendar. They don't seem as slick as eventSherpa. There should be some way to get Outlook to export an iCal file as well but that seems to be an add-on feature.
A cool option with iCal, is to publish it to a server and then use PHPiCalendar to display it. Check out the demo. This could be a great combination for schools and volunteer organisations.