Available for both KDE3 and KDE4, KTorrent is a fully-featured, complete and powerful client for the BitTorrent protocol. The latest version, 3.2.1, was released earlier this month and includes a lot of bug fixes and web seeding improvements.
KTorrent has been always the most popular client at least on KDE, including all the configuration options needed and a reasonable number of useful plugins to extend its functionality up to a full-blown BitTorrent client.
One of the most useful plugins is the ability to search on various torrent sites directly from within the application, without the need of an external web browser.
Searching on torrent sites directly from KTorrent
You can also easily add custom torrent sites if you're not satisfied with the default ones. Each search result is opened in a new tab inside KTorrent. Among the other plugins included are the DownloadOrder plugin (which allows to specify the order in which files belonging to the same torrent are downloaded), InfoWidget plugin (which will display useful information about a torrent in separate tabs), or the Syndication plugin, which supports both RSS and Atom feeds. These are not all though, you can enable or disable plugins via the Settings -> Configure KTorrent -> Plugins menu.
KTorrent will allow to download only certain files from a torrent, set bandwidth limits, it has system tray integration, it provides notifications, customisable keyboard shortcuts just like any respectable KDE application, and it also includes a configurable queue manager.
Just like Deluge, it offers a pretty easy-to-use interface and will display by default information about the torrents which are downloaded at the bottom, in several tabs. It shows the status, the torrent files, peers, chunks and even trackers.
You can also create new torrents and eventually share using trackers which don't require registration, like http://open.tracker.thepiratebay.org/announce or http://denis.stalker.h3q.com:6969/announce.
Creating a new torrent
As a conclusion, I find KTorrent to be one of the most powerful clients for the BitTorrent protocol available for Linux, and definitely the best for KDE users. It gets new features and improvements with every release and it also supports trackerless torrents.