Mac only: uTorrent, the most popular BitTorrent client on Windows, has moved from alpha to beta for OS X, boasting a completely native Mac client with a brand new interface.
Most Mac users are probably happy with Transmission, but the lightweight uTorrent is definitely worth a look — not only is the new UI much better than the old one, it's much nicer than Transmission's if you download a lot of torrents at once. Furthermore, uTorrent can definitely compete with Transmission on lightness, while still having some of the more advanced features of something like Vuze (though Transmission continues to gain advanced features with each update). If you're a recent Mac convert looking for a more familiar torrent client, or just a heavy downloader unhappy with Transmission's more minimal UI, the new uTorrent beta is definitely worth a look.
The uTorrent beta is a free download for Intel Macs running 10.5 Leopard or higher.
uTorrent Beta for Mac [uTorrent]