rTorrent is a popular command line based bittorrent client. It provides very powerful features yet it is very light on the system - contrary to other bittorrent clients like Vuze. There are a couple of webinterfaces for it but they are not truly needed. SSH access combined with the "screen" program provide you all the tools you need.

I have a constantly running server that would be a perfect on which to run torrent downloads. The only catch is that the server is headless -- it has no monitor or keyboard. To overcome this obstacle, I use rTorrent as my torrent client, and GNU Screen so that I can disconnect my terminal session and leave rTorrent running.