BusySync is a preference pane that allows you to synchronize iCal calendars across multiple computers without the need for a .Mac subscription. It utilizes many core Mac OS X technologies such as SyncServices, CoreData, and Bonjour, and the result is a pretty slick alternative for those who use .Mac primarily for its calendar synchronization capabilities.
BusySync uses peer to peer calendar sharing rather than .Mac's client-server model. In the preference pane, you can select which of your calendars you would like to share, and then select what permissions you'd like others to have (read, read/write, administrative, etc). It will automatically discover shared calendars over local Bonjour networks and automatically synchronize updates as they are made. However, users not daunted by port forwarding can also make their calendars accessible via the internet for sharing. SSL encryption is available (and advisable) to users of Mac OS 10.5 Leopard.
BusySync 2 adds Google Calendar synchronization support. In a demo, John Chaffee of BusyMac demonstrated how a change on Google Calendar was quickly reflected on each of his iCal calendars on his two notebooks in his booth, and vise-versa. He also mentions that Google Calendar support will also be useful to mobile users who want to have a read/write copy available on the internet, something that .Mac does not currently support.
BusySync 1.5 is available for $20 per computer, though discounts are available for multiple users. BusySync 2, when it comes out, will be $25, however if you buy BusySync 1.5 now, you will be eligible for a free upgrade. A time-limited demo version is also available via BusyMac's website.