The Apple Blog has a post that mentions that a new version of Mozilla Sunbird has been released. This is a pretty good looking calendar application. It has the same functionality as Apple's iCal, but also some additional features:
- Allows you to modify calendars that you are subscribed to. So now you can have multiple computers modify shared calendars.
- Support recurring to-do items
- Multi-week view
I don't think that I will use this for my primary calendar software. The one feature that is missing is syncronization with iSync. Maybe someone will write an extension that will allow iSync capability.