Deluge is a fully-featured cross-platform BitTorrent client. It is Free Software, licensed under the GNU GPLv3+ and adheres to freedesktop standards enabling it to work across many desktop environments.
GTK-Gnutella
gtk-gnutella is a server/client for the Gnutella peer-to-peer network.
Tixati
Tixati is a New and Powerful P2P System