A Bittorrent client using C++ / libtorrent and a Qt4 Graphical User Interface. It aims to be a good alternative to other bittorrent clients. It has more and more features such as an integrated search engine, UPnP, encryption, PeX, a torrent creation tool, more.
Features:
Polished µTorrent-like User Interface
Well-integrated and extensible Search Engine
Simultaneous search in most famous BitTorrent search sites
Per-category-specific search requests (e.g. Books, Music, Movies)
All Bittorrent extensions including DHT, Peer Exchange, Full encryption, Magnet/BitComet URIs.
Remote control through a Web user interface
Nearly identical to the regular UI, all in Ajax
Advanced control over trackers, peers and torrents
Torrents queueing and prioritizing
Torrent content selection and prioritizing
UPnP / NAT-PMP port forwarding support
Available in ~25 languages (Unicode support)
Torrent creation tool
Advanced RSS support with download filters (inc. regex)
Bandwidth scheduler
IP Filtering (eMule and PeerGuardian compatible)
IPv6 compliant

Home Page - http://qbittorrent.sourceforge.net/