BitTornado has to be the most minimalistic BitTorrent client available. This application has a sober interface by means of which we'll be able to follow the status of our downloads. To do so, once we have the desired torrent file, all we will have to do is open it with the program, that will immediately start searching for the necessary sources to download it.
Really simple alternative to uTorrent
This P2P download manager, the heir of Shad0w's Experimental Client, stands out mainly due to the low resource consumption and how easy it is to use. In fact, it hardly includes any configuration options: it's only possible to limit the upload and download speeds, to give priority to some downloads over the rest and to view the information about the download that is currently taking place. The program will also allow us to encrypt our transmissions.
All in all, if you're ready to sacrifice the aesthetics and the possibility to configure them that other BitTorrent managers offer in pursuit of low resource consumption, choose BitTornado, the most simple torrent file download manager to date.