Tribler is an open source peer-to-peer decentralized BitTorrent client with various features for watching videos online.
Tribler covers basics such as Prioritization, Selective Downloading, DHT, PEX and Magnet URIs as well as Broadcatching and Search. It includes MSE/PE for security. It has graphical, web and command line interfaces. Has UPnP, NAT-PMP, NAT traversal for automatic router configuration. Has µTP data transfer protocol support. Even covers web seeding and embedded tracker. And it auto-updates.