Rendezvous is Apples (now open-source) implementation of the Zero-Conf open networking standard. Basically it has 2 main prongs:
1) You can add computers to a network and have them automatically negotiate an IP address without the need for a DHCP server or any human intervantion.
2) The computers can advertise services on the network and other computers can search and use these services without any configuration of the network.