One of the features that I have think has been missing from iSync and .Mac is the ability to sync a subset of my address book with my wife's computer. She doesn't want to have everyone that is in my address book in hers, and I don't want a complete copy of hers either.
I would have hoped that with the .Mac syncing of the Address Book, I could specify only one group that should be sync'd to each computer. But it looks like Apple's implementation will only allows you to sync the whole address book between each computer and the .Mac servers.
address-o-sync is a solution for this problem. This application allows you to specify either your whole address book or one group that can be sync'd across the network.
This application uses Rendezvous for broadcasting itself over the network, and it allows you to specify a password that is required in order to perform a sync.
This work very well, as it allowed me to sync one of the groups in my address book with my wife's address book, and it lets Address Book take care of the conflict resolution.
The only problem that I had was that my firewall was blocking any incoming connections. I added port 9876 to the list of ports that will accept an incoming connection.