JTella is the JavaAPI for the GNUTella network. Using JTella, Java based GNUTella tools and servants can be created, with JTella providing the network interface code.
FiShNet is a P2P file sharing application written in Java. It has a server as well as client so that it can be used to create private file sharing network.
Jockey is a framework API to abstract the details of a peer-to-peer network and/or a service location/invocation framework from an application programmer.