About BusyCal
Share calendars on a local area network and over the internet without the need for a dedicated server. Shared calendars can be viewed and edited by multiple users and changes are instantly synchronized between users. BusyCal also syncs with Google Calendar offering online access and sharing cross platform.

In addition to calendar sharing, BusyCal provides the ability to enter and view event details in a non-modal floating window or an embedded info pane; create To Dos, including recurring To Dos, that display in the calendar and auto-forward until completed; display events in several customizable views including a List View and scrolling Month and Week Views; many more features.

Read more: http://www.apple.com/downloads/macosx/productivity_tools/busycal.html