The qBittorrent project aims to provide a Free Software alternative to µtorrent. Additionally, qBittorrent runs and provides the same features on all major platforms (Linux, Mac OS X, Windows, OS/2, FreeBSD).

qBittorrent 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
        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 ~41 languages (Unicode support)
Torrent creation tool
Advanced RSS support with download filters (inc. regex)
Bandwidth scheduler
IP Filtering (eMule and PeerGuardian compatible)
IPv6 compliant
Sequential downloading (aka "Download in order")
Available on most platforms: Linux, Mac OS X, Windows, OS/2, FreeBSD