A Zope calendar with the possibility to easily make personal calendars and group calendars, with iCalendar import and export, meeting support, recurring meetings, full internationalisation support and more.
This ZopeProduct is the basis for the calendar in CPS and for the CalCMF? product for CMF and Plone.
http://www.cps-project.org/sections/projects/calendar_server
http://plone.org/products/calcmf