Which torrent apps work headless (from command line only)?

"You should look at deluge. It is cross-platform, and is excellent as a headless app.

Deluge is a full-featured BitTorrent
client for Linux, OS X, Unix and
Windows. It uses libtorrent in it's
backend and features multiple
user-interfaces including: GTK+, web
and console. It has been designed
using the client server model with a
daemon process that handles all the
bittorrent activity. The Deluge daemon
is able to run on headless machines
with the user-interfaces being able to
connect remotely from any platform.

Deluge features a rich plugin
collection; in fact, most of Deluge's
functionality is available in the form
of plugins.

Deluge was created with the intention
of being lightweight and unobtrusive.
It is our belief that downloading
shouldn't be the primary task on your
computer and therefore shouldn't
monopolize system resources.

Deluge is not designed for any one
desktop environment and will work just
fine in GNOME, KDE, XFCE and others.
We do our best to adhere to the
freedesktop standards.

I have the deluged daemon run as a service on boot, and also run a client on the same machine that hosts the web interface. I then use the web interface from all my systems."

For Unix-based systems you can use rTorrent.

"It appears that you have many options.
According to this page and my knowledge, you can use any of these:

qbittorrent-nox
Deluge
aria2
Transmission (daemon)
rtorrent"