BitTorrent
P2P protocol for distributed file sharing. Bram Cohen, 2001. Files shared via a "swarm" of peers instead of a central server. Efficient for large popular files.
.torrent file (or magnet link) → tracker (or DHT, distributed hash table) → swarm. Tit-for-tat algorithm prevents freeloading. Drove ~30% of internet traffic around 2010, has shrunk with streaming services. Legitimate use cases: Linux distro ISOs, World of Warcraft patches, Facebook's internal deploys, Internet Archive. Webtorrent (2014) brought BitTorrent to the browser via WebRTC. IPFS is a spiritual successor with content addressing.