aria2 is a utility for downloading files. The supported protocols are HTTP(S), FTP, BitTorrent, and Metalink. It has a powerful segmented downloading ability, downloading a file from multiple sources and multiple protocols and utilizing your download bandwidth to the maximum. It supports downloading a file from HTTP(S)/FTP and BitTorrent at the same time, while the data downloaded from HTTP(S)/FTP is uploaded to the BitTorrent swarm. Using Metalink's chunk checksums, aria2 automatically validates chunks of data while downloading a file like BitTorrent.
SALI (Sara Automatic Linux Installer) is a tool that allows you to install Linux on multiple machines at once. It support several protocols for downloading by way of aria2 to install a machine. For example, BitTorrent and rsync are supported. SALI originates from SystemImager and still uses the same philosophy. It is a scalable method for performing unattended installation. SALI is mostly used in cluster setups.
PeerProject is a versatile peer-to-peer filesharing client and file manager. It's intuitive and aesthetic while supporting many networks and features, including BitTorrent, G2, Gnutella, eDonkey, DC++, IRC, HTTP/FTP, and more. It's a mainstream fork of Shareaza p2p, for Windows and WINE.
KTorrent is a BitTorrent application that allows you to download and share files using the BitTorrent protocol. Key features include queuing of torrents, global and per-torrent speed limits, previewing of certain file types, importing of partially or fully downloaded files, file prioritization for multi-file torrents, selective downloading for multi-file torrents, kick/ban peers with an additional IP Filter dialog for list/edit purposes, UDP tracker support, support for private trackers and torrents, support for µTorrent's peer exchange, support for protocol encryption (compatible with Azureus), support for creating tracker-less torrents, support for distributed hash tables (DHT), support for UPnP to automatically forward ports on a LAN with dynamic assigned hosts, support for webseeds, scripting support via Kross, and interprocess control via DBus interface.
PyroScope is a collection of tools for the BitTorrent protocol and especially the rTorrent client. "pyrocore" is the component providing command line tools for automation of common tasks, like metafile creation, and controlling rTorrent via its XML-RPC interface. It also offers a documented API, with which these tools are implemented.
PHPTracker is a PHP library to generate .torrent files, set up a Bittorrent tracker server, and a seed server too. It is capable of creating standard .torrent files from any file on your server, tracking client peers and managing peer announcements, and initially seeding your file with a peer daemon that automatically shuts down once you have enough external seeders.
pytvshows downloads torrents for TV shows from RSS feeds provided by ezrss.it (previously tvRSS.net). It is designed to be run from cron or a shell and interfaces with BitTorrent clients like Transmission (via the Transmission RPC API) or any other BitTorrent client that supports folder monitoring (most). This is a resurrection of the venerable pytvshows script by Ben Firshman.