We’ve mentioned Calgoo Hub – a calendar sharing application that handles a variety of formats – before. They’ve just introduced a small but significant upgrade in the form of CalDAV support. This means that you can now directly publish and share calendars from within Outlook 2007, iCal, and Sunbird without any plugins.
In addition to those three, Calgoo Hub supports sharing and subscribing to calendars from Outlook 2003, Google Calendar, 30 Boxes, and Calgoo’s own flagship Calendar product. Several other applications can subscribe to updates. The main hurdle for some people will be the Hub software itself, which only runs on Windows.