How can I sync iCal between multiple Macs?
You can share calenders using the publish option in iCal. You can do this on Mobile Me (for which you need a subscription) or on another server, for example Box.net (which is free). However, if you share a calenders this way, only the owner of the calender can edit it, other subscribers can only view it.
I personaly very much like Spanning Sync. This application uses Google Calender to sync iCal calenders between multiple Macs. And the calenders can be edited on all those multiple Macs as well. It’s not free, but you can get a subscription for $25 a year or $65 forever. And one subscription can be used on multiple macs. And all updates are included with the subscription (currently they are working on contacts syncing for the 2.0 version).
An alternative to Spanning Sync might be Busy Sync, which is targeted more to syncing calenders on a LAN, but does include an option for syncing with Google Calender as well. Busy Sync costs $25 per computer.
Update: might also check Calgoo.
Publish calenders with Box.net: tested it, works
Spanning Sync: tested it, like it a lot
Busy Sync: did not test it
Calgoo: did not test it