command-line tool for a single download of a torrent (like wget or curl)

Check out transmission-cli. The usage is as simple as running transmission-cli <torrent-file>, but you can obviously tune it to your needs with several options.

Just a side comment:

Actually you could use many other options, apart from transmission-cli and there will probably appear many other suggestions here (like deluge by Benjamin B. in the comments). I've read somewhere that any well-behaved program should be written so that it can be controlled via command line and the GUI is only an addition to that -- an interface to make the program easier or more convenient to use.

aria2

aria2c <torrentfile or magnet link>

You can also try rtorrent and may be ctorrent which man page starts with:

ctorrent - Download bittorrent files from command line