The latest available source of BitTorrent was published in 2009 at BitTorrent website as BitTorrent-5.3-GPL.tar.gz under GPL/GNU license.
It's version 5.3, however other version between 2.9.5 and 5.3.0 are also available.
You can checkout this version from GitHub (with full release history). Also available here.
Alternative open-source projects includes:
- WebTorrent - a streaming torrent client for node.js
