You mean a
client as they are known in the business? If you're on GNU/Linux i would highly recommend KTorrent, but Azureus is more popular.
If you're on Windows, uTorrent is (as far as i know) the best option. Be sure to run Peer Guardian to keep the RIAA from trying to stop you downloading.........Linux distros
If you mean a
tracker (i think that's the right word) as in a site with all the .torrent files for you to download them; then I'd recommend Demonoid.com (pm me if you want an invite) if you're going to be using bittorrent alot. If it's just for a couple of things then just use sites like isohunt.com and mininova.org