Why Should It Be Included?: While Tribler uses the BitTorrent protocol to download and upload files, and is fully compatible with most BitTorrent clients, it adds a number of important extensions: Fully integrated content search; Support for Merkle hashes; An overlay swarm for communication between (Tribler) peers; The ability to cooperatively download. In short Tribler is the evolution of P2P technology building on but also going beyond the successful BitTorrent protocol. Tribler offers built-in tor-like encryption so that VPNs are no longer necessary for torrent downloading/uploading.
Is It Open Source?: Yes
How Many Users?: Their Specification of Tribler Anonymity page estimate that over a million users will use the software.
Source Tarball: https://github.com/Tribler/tribler (Under "Releases" are versioned tarballs.)