qBittorrent is a multi-platform lightweight but fully featured BitTorrent client. It's authors claim it's the closest open source (GNU GPL v2 license) equivalent to µTorrent.

There are a lot of good BitTorrent applications for Linux, but some people just don't want to give up uTorrent because either they are too used to it or just because it actually works quite OK in Linux, under Wine. One major problem with using uTorrent this way is that Firefox doesn't open .torrent files with uTorrent, so you have to save the torrent file and then load it.