Deluge is a full-featured BitTorrent client for Linux, Mac OS X and Windows. (GPL)

FrostWire is a peer-to-peer file sharing program for the Gnutella and BitTorrent protocols. FrostWire is written in Java, and is a fork of LimeWire. (GPL)

KTorrent is a BitTorrent client for KDE and is released under the GNU GPL License. (GPL)

GTK+ BitTorrent client (GPL)

qBittorrent is an advanced P2P bittorrent client programmed in C++/Qt4. (GPL)

Transmission is a fast, easy, and free multi-platform BitTorrent client. (GPL)

Vuze implements the BitTorrent protocol using java language and comes bundled with many invaluable features for both beginners and advanced users. (GPL)