Deluge is a simple and lightweight bittorrent client with a very extensive web interface. Since discovering it a year or so ago, I haven't used anything else. I fully recommend giving this a go.
You can password protect the web interface and also set the GTK interface to require password when you 'unhide' it from the system tray. Many other options available. Many distributions have the webui plugin as a separate package. Check with your distro's package manager.
Official Website: [size=78%]http://deluge-torrent.org/
Any questions? Need help with Deluge? Please comment below.