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qBittorent is an open-source software alternative to µTorrent.
It runs and provides the same features on all major platforms (Linux, Mac OS X, Windows, OS/2, FreeBSD).
The app was recently updated to v4 and among the several new features is the use of flat SVG icons, making the UI look better and more native to more recent OS's.
qBittorent V4
The Open Source uTorrent alternative just got a new look
qBittorrent
Free and reliable P2P Bittorrent client