gtk-gnutella!

Phex is a peer-to-peer file sharing client for the Gnutella network. It is free software and without spyware or adware. Phex is released under the GNU General Public License. Phex is based on Java technology and therefore available for many different systems that offer a Java 1.4 runtime environment, such as Windows, Mac OS X, Linux and others.

BeShare is a file-sharing program for BeOS. You can use it to swap files with your fellow BeOS users. If you've ever used Napster or Gnutella, you have a pretty good idea of how it works.