Julienne is a PyGTK BitTorrent client. It's only tested on linux, but should work anywhere Bittorrent, PyGTK and Gconf, and Python run.
Its distinguishing features are its simple interface, and it's front-end / back-end separation. The first means it doesn't follow the GNOME HIG, but shouldn't be hard for you to use anyway. The second means you can close the graphical interface, and let your file transfer will continue in the background.
Download it direct from its place in my subversion repository, or check out one you can update later (svn up) or create patches (svn diff > julienne.diff) with the command: svn co http://www.tortall.net/svn/mu/trunk/julienne.