Marvell recently introduced the Plug Computer. It's a small and energy efficient server, which you just "plug" to a power outlet and your network.
I really like the price: They sell a developer kit for 99,- USD. If they're right about power consumption, it would cost around 10,- € a year to run this server 24/7, even if a USB-Stick is attached to extend space.
I really want one. I always hate it when downloads don't finish and I can't switch my computer off until the job is done. It would be great to have a small Firefox plugin which redirects all downloads to the server and a small cron job which syncs all server contents to my desktop.
It might even be a good business opportunity. How much would you pay for such a solution?