Transmission is a free, lightweight BitTorrent client. It features a
simple, intuitive interface on top on an efficient, cross-platform
back-end.

Transmission is open source (MIT license) and runs on Mac OS X (Cocoa
interface), Linux/NetBSD/FreeBSD/OpenBSD (GTK+ interface) and BeOS
(native interface).

WWW: http://www.transmissionbt.com/