I have an ASUS RT-N16 router at home, which I connected to a hard-drive by USB. Then with Tomato installed, you can SSH into the router and install optware. Then you can configure optware[1] to install its packages on the external hard-drive. From there, you can retrieve and install a bunch of packages, like http servers, bittorrent clients and whatever else you feel like running from the CLI. Pretty convenient! I installed Transmission with a web-gui and I can SSH into it from anywhere in the world to my house's router, load a magnet or torrent in the queue and when I get home, the content will be waiting for me on my hard-drive.

I don't pretend Tomato firmware are the first to allow to do so, but I just found it was an interesting and usefull thing.

[1] http://tomatousb.org/tut:optware-installation