Sunbird is a standalone calendaring application from Mozilla. New builds for Windows, Linux and Mac OS X were released on Aug 17th.
Screenshots are available of the application – which is still very much in Beta mode. It uses the same format (rfc 2445) as and appears very similar to Apple’s iCal application.
Seems like an app to watch out for – esp. if you have access to a WebDAV server and want to publish your calendar. This seems like a more direct approach to calendaring than the RSS Calendar option discussed previously.