UC San Diego and Microsoft Research have come up with a slight variant on a network card.
This variant is essentially a computer with limited processing power and flash memory. It could be used to perform limited tasks (such as downloading torrents and listening for VOIP calls) while the host PC is powered down.
It can either perform these functions itself (assuming code has been written to enable it to do so), or act as a sort of Wake On Lan on steroids, responding to all sorts of events, rather than just a "Magic Packet" sent to the local subnet (the standard WOL signal).
http://www.reghardware.co.uk/2009/04/27/somniloquy/