Spanning Sync is a very handy piece of software that provides long-sought functionality - two-way synchronization between iCal and Google Calendar! The idea seems to be something similar to .Mac's syncing between two macs. Unlike .Mac, however, Google Calendar can be written to via web interface. This is a huge feather in Google's cap, and allows this functionality to be so useful. Stuck on a PC, or just a computer you don't want your password in the keychain for, at the office? No problem. Just add an event to your google calendar and Spanning Sync will bring it down to your mac at home for you.

Beware, though - the software is in beta and should be treated with caution until its full release.