qBittorrent - Lightweight and open source torrent client that runs on all major platforms.
Rust - A safe, concurrent, practical language
Transmission - Transmission has the features you expect in a BitTorrent client: encryption, a web-based interface...
Haskell - An advanced purely-functional programming language
BitTorrent - BitTorrent is a peer-to-peer program developed by Bram Cohen and BitTorrent, Inc.