Came across an interesting paper today. What if you could reduce the avg. power usage of your computer from ~200Watts to ~200mWatts when you are not using it? I usually keep my system on for downloading torrents and this is exactly what makes it possible to put everything but the network interfaces in your computer to sleep.
Enter Somniloquy, which means to talk in one’s sleep. The computer is theoretically sleeping but can network and download data. For this they use a modified network card with a low power embedded microprocessor and a data card. This processor takes care of the ongoing downloads when you put it in a semi-sleep state. Once the miniature 2 Gb SD card is filled up, you wake up the computer to copy the data onto the hard disk and put it back to sleep and download.
An interesting read. You can get the paper here.