BusySync is yet another application that will make you say, "Why didn't Apple think of that?" BusySync makes it extremely easy to sync up multiple iCal calendars between different computers on a home or office network, without any need for a central server. BusySync is easy to set up, and users can view and edit shared calendars, instantly and automatically publishing any changes--plus you can simply sync peer-to-peer, between different Macs, or even sync between a combination of individual Macs or calendars hosted on a server.
Even better, BusySync syncs with Google Calendar, too, so that means you can view, edit, and sync calendars online, from anywhere (and power users can also sync using remote access over the Internet). This lets you synchronize calendars between home and work, and any changes made offline are synced the next time you connect. BusySync also lets you put password-protection on certain calendars, and limit access to particular users. Overall, this is already a useful app and a good value, but current users will also get a discount if they convert to BusySync's upcoming big-sister app BusyCal.