BusySync is a terrific utility that makes sharing iCal calendars easy across networks or the internet (we first wrote about BusySync in September). Simply install the preference pane on all participating Macs, and select the calendars you'd like to share. The next time you update a given calendar, your changes will appear across all machines. You can also make your calendars read-only or editable by everyone.
I've been using it at my day job across departments and it's great. Version 1.5 adds internet synchronization. You can read the change log here.
BusySync requires Mac OS 10.4 or later and costs $19.95 for a single license (multi-license bundles are available).