Are there any torrent clients that run as a service?

I turn off all of my computers at night except for my server. I'd like to be able to have my torrent downloads run on my server, which won't be logged in, so I'm wondering if there are any clients that can run as a service.

I'm ideally looking for one that can be passed a path to a .torrent file and will begin downloading it (I'll write the program that will interact with it).

uTorrent definitely works as a service (I use it), and there's guides out there (need I say google?) if you have some preference of client, try searching "install some_bittorent_client as a service"

Look at Hadouken, it is a BitTorrent client for Windows which runs as a Windows Service. It is also free and open source.

Disclaimer: I am the author.