FreeNAS server has a very light and fast torrent downloader called Transmission, it’s multiplatform and opensource; it came with daemon version and you can find more info on Transmission web site.
You can find many addons for manage Trasmission, and I found Trasmission Remote very nice, it’s a .NET app that seems like uTorrent, my first choice torrent downloader for a single PC download, really I think UI is a uTorrent clone for Transmission, and it’s opensource.
Here a snapshot taken from project site:
