rTorrent is a popular command line based bittorrent client. It provides very powerful features yet it is very light on the system – contrary to other bittorrent clients like Vuze. More here

Recently a large bittorrent tracker announced, that they will in future only support magnet link. Magnet links are special URLs that contain a hash link that identifies the according .torrent file. By default, rtorrent does not support magnet links. However there is a patch for it provided by Josef.