
Many of today's calendars do this such as Apple iCal, Google Calendar, Yahoo Calendar, Thunderbird, and many more as well. Microsoft Outlook 2003 nd 2007 require an add-on for 2-way sync. We'll see what they do next version.
By simply sending your friends and family the required CalDav “dotCalConnect” information, your family and friends may be able to directly use their current Desktop & Webtop calendars, or dotCal, to fully sychronize with the calendars you selectively make available to them.
What a way to CAL-municate!!