I know this may be a rather controversial section but as long as people will torrent then there will still be programs around supporting it.  The program(s) listed below can be used for legal or illegal downloading of files.  I do not purposefully promote the use of illegal torrents and cannot be held responsible if legal proceedings are brought against you.  You have been warned.

µTorrent is by far the best BitTorrent client.  It provides easy setup for even the most computer illiterate folk with its speed guide.  Now owned by BitTorrent itself µTorrent has formed the basis for their own client and both applicatation are one in the same.
Download: http://www.utorrent.com/downloads