CalendarX is the most robust Plone calendar application (we've tested a few). It has been around since the days of Plone 1. It manages vanilla or custom Plone events and is highly customizable. The product has excellent separation from the content it manages, something we've learned to like a lot in a Plone product. It is available for both Plone 2 and 3. The Plone 3 version is much improved for customization.