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. This howto works for Ubuntu 9.10 Karmic Koala and for Debian 5 Lenny.