Windows - µTorrent
It is a shame that they put ads in now, failing to live up to its name, but still a very good, modern, full-featured client.
UNIX (Headless) - Deluge daemon + deluge-web web interface.
Large media companies routinely download peer lists from tracker sites for torrents of their content, and send cease-and-desist letters to the ISPs of the peers. ISPs will usually implement some type of sanctions against customers for which they receive these letters. Usually this involves shutting off the customer's internet until they acknowledge their alleged wrongdoing and sign an agreement to refrain from it in the future.
Currently, tracker peer lists are the only known method media companies use to find alleged copyright infringers. Trackers are also unnecessary for most torrents these days. DHT, and other decentralized peer discovery methods are entirely sufficient for high-performance torrenting. Disable the use of trackers in your torrent client, and you will be golden.
If you must use a tracker, it might be possible to conceal your identity by having your client report a false IP address, though I have not researched or tested this.
In theory the media companies could just join the DHT swarm and harvest addresses, but none are known to do this yet (2013) and likely never will, because its just too darn easy and effective to download tracker lists.
Preferences > BitTorrent, uncheck
Enable UDP Tracker Support, and click
OK.. You should be safe now, but to play it safe I also right click on each torrent in the queue and go to
Properties, delete everything in the
Trackers box and click