Though BitTorrent has received a bad rap as the latest vehicle for piracy, it also has legitimate uses. This is especially true when dealing with FOSS. Since many Linux distros and FOSS apps are often developed, curated, and/or hosted by individuals or groups of volunteers, using the BitTorrent protocol can seriously cut down on monthly bandwidth expenditures. Even projects backed by large corporations, like openSUSE (Novell) and Fedora (Red Hat), are utilizing peer-to-peer (P2P) file sharing as a way to cut down on hosting fees and save bandwidth. Also, as time passes, more and more movies, music, books, and images are losing copyright protection and entering the public domain, making them perfectly legal to download and share with others.
KTorrent (v. 3.2.1)
KTorrent is not the BitTorrent client for KDE, at least officially. KGet is actually the official KDE download manager, and it also handles BitTorrent files. Despite this, most KDE distributions use KTorrent as their default BitTorrent client. Why? Because it is awesome. What really sets KTorrent apart from the pack is a mind-numbing number of ways to track statistics.
Deluge (v. 1.1.6)
Deluge is like a much more robust version of Transmission (a popular distro-default). Deluge contains many tools for queue management and statistics tracking. If you are a heavy BitTorrent user, yet still enjoy a clean and simple interface, consider giving Deluge a try.
Transmission (v. 1.51 b7963)
Transmission is simple and compact, but definitely still a class above the lightweight clients. Yet, it lacks some of the advanced features of KTorrent, Deluge, and qBittorrent. It doesn't have a ton of options, details, or statistical tools, but if it came pre-installed on your distro and you only do occasional (legitimate) BitTorrent transfers, there is no reason to install a different client. Transmission is the perfect middle-ground, which is probably why it's the default client on many popular distros, including Ubuntu.
qBittorrent (v. 1.3.3)
qBittorrent has a very slick tabbed interface with glossy KDE-esque controls. The best feature of this app is the search tab. Here you can search for torrents on several pre-loaded torrent tracking sites, all from the same search box. You can also customize the search function by adding and/or removing torrent trackers from the list to search. Also, multiple searches can be done at the same time; new searches will simply open in new tabs. With the Pirate Bay in limbo, the search feature makes qBittorrent even more attractive. Other features include RSS support, IP filtering, and proxies. Unfortunately, the bandwidth throttling and upload connection limits are ignored. Otherwise, this app is a serious contender for the top spot.
Vuze (v. 22.214.171.124)
Vuze is actually one of the oldest BitTorrent clients; it began under the name Azureus. The main reason that Vuze is so low on this list is due to Vuze Updater, its version updating system. Once you run Vuze, it will check for updates and proceed to download and install them. In my experience, it cannot seem to complete the updating process to version 126.96.36.199, and continually asks for the same update whenever you run the application. You can ignore the prompt to update in order to get around this (which you must, since it requires a restart to “complete” the update). The problem with that solution is that the user will eventually become accustomed to ignoring updates.
Also, the Linux edition of Vuze is actually an older version than the current version for Windows. As a result, the new Vuze HD Network features are not yet compatible with Linux. All of that aside, Vuze gives new users the option between a beginner, intermediate, or advanced interface and feature set, making it an appropriate BitTorrent client for any user.
Monsoon (v. 0.21)
Monsoon has a similar interface to Deluge, but without the massive number of options–-think Transmission's options with the interface of Deluge. What really makes Monsoon stand out from the other BitTorrent clients is the inclusion of an RSS manager.
BitTorrent (v. 4.4.0)
The original BitTorrent client is called--surprise--BitTorrent.
If you like your apps lightweight, BitTorrent clients don't come much more discreet than BitTorrent. This app has a minimal number of options. It simply handles torrent uploads and downloads; there are no advanced tracking or queuing features.