Stocking up on the Internet before I spend a week on holidays:
JmDNS is a Java implementation of multi-cast DNS and can be used for service registration and discovery in local area networks. Part of JTunes.
JXTA 2: A high-performance, massively scalable P2P network "JXTA 2 introduces the concept of a rendezvous super-peer network, greatly improving scalability by reducing propagation traffic...Implementation of the shared resource distributed index (SRDI) within the rendezvous super-peer network, creating a loosely consistent, fault-resilient, distributed hash table".
Both ripe for the impending release of the Kowari project on SF.