Deluge is a lightweight, free, cross-platform BitTorrent client. It uses libtorrent in its backend and PyGTK for its user interface.
Deluge features a rich plugin collection; in fact, most of Deluges functionality is available in the form of plugins. Deluge was created with the intention of being lightweight and unobtrusive.
Features:
Web UI
BitTorrent Protocol Encryption
Mainline DHT
Local Peer Discovery (aka LSD)
FAST protocol extension
Torrent Peer Exchange
UPnP and NAT-PMP
Proxy support
Web seed
Private Torrents
Global and per-torrent speed limits
Configurable bandwidth scheduler
Password protection
RSS
And much more!
Linux and Mac OS X versions of Deluge are available at the developers website.