Free. Windows. Aside from being a peer-to-peer file transfer protocol, BitTorrent is also the name of the official software used to connect to the BitTorrent network. BitTorrent hadn't seen a major update in about two years, but that was before it bought out uTorrent.
Free. Windows. If you're looking for a good BitTorrent client, try uTorrent. It has what other BitTorrent clients have, such as scheduling, bandwidth management, and Mainline DHT, plus extras like a unique protocol that detects and corrects for heavy traffic.