qBittorrent is the closest open source (GNU GPL v2 licensed) equivalent to
popular uTorrent.  It is based on the Qt4 toolkit and libtorrent-rasterbar.

Features:

  - 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: DHT, Peer Exchange, Full encryption,
    Magnet URI, uTP
  - Remote control through a Web user interface (nearly identical to
    the regular UI, all in Ajax)
  - Advanced control over trackers, peers, and torrents: queueing and
    prioritizing, content selection and prioritizing
  - UPnP/NAT-PMP port forwarding support
  - Available in ~25 languages (Unicode support)
  - uTorrent spoofing to bypass private trackers whitelisting
  - Advanced RSS support with download filters (inc. regex)
  - IP Filtering (eMule and PeerGuardian compatible)

WWW: http://www.qbittorrent.org/