Deluge is a full-featured BitTorrent client for Linux, Mac OS X and Windows. (GPL)
Linux - FreeBSD - Windows - macOS
FrostWire is a peer-to-peer file sharing program for the Gnutella and BitTorrent protocols. FrostWire is written in Java, and is a fork of LimeWire. (GPL)
Cross-platform
Transmission is a fast, easy, and free multi-platform BitTorrent client. (GPL)
Linux - FreeBSD - macOS
WebTorrent is a streaming torrent client for the web browser and the desktop. It's written completely in JavaScript – the language of the web – and uses WebRTC for true peer-to-peer transport. No browser plugin, extension, or installation is required. (MIT)
Cross-platform
4 applications