Deluge

Deluge is feature packed, and any features it doesn't already have can be added with additional plugins. On its own it already supports:

• Web UI
• BitTorrent Protocol Encryption
• Mainline DHT
• Local Peer Discovery (aka LSD)
• FAST protocol extension
• µTorrent Peer Exchange
• UPnP and NAT-PMP
• Proxy support
• Web seed
• Private Torrents
• Global and per-torrent speed limits
• Configurable bandwidth scheduler
• Password protection
• RSS

Deluge is Linux, Mac OS X and Windows.

Deluge