A PeerToPeer project by SunMicrosystems, see http://www.jxta.org/.
The project was started by BillJoy
The technology is pretty clever, but suffers from a lack of documentation and the APIs have many rough edges. People therefore tend to become infuriated by it rather quickly. With a bit of effort in cleaning up the APIs and documenting them, Jxta could become one of the key Java technologies.
Jxta has its own wiki at http://wiki.java.net/bin/view/Jxta/WebHome
Some Jxta Projects that use Jxta are:
CategoryJava CategoryDistributed CategoryCollaboration CategoryProject