Microsoft Research in partnership with USCD are working on a USB device, called the "Somniloquy" (which by definition means to "Talk in one's sleep") which allows your computer to carry on performing network processes while it is asleep or in a low power state.
The Somniloquy allows a computer to process network traffic, even if the computers CPU, hard disk, display and I/O buses are powered down, without losing network connectivity. As described in the research paper, the interface could manage many network-related tasks such as maintaining a remote desktop connection, receiving instant messages, torrenting and managing a VOIP account, while the computer itself is asleep.
The USB network adapter (as described in this research paper) is made up of a Gumstix chipset with a 200 MHz XScale processor, 64 MB of RAM and a 2 GB SD memory card running Linux. If the adapter detects that the connected machine has entered sleep mode, it copies over the networking information and begins carrying out communications on its behalf.
The Microsoft and USCD researchers also showed that the adapter can perform more complex tasks - like the mentioned instant messaging application. The example in the research paper shows that the researchers created a modified instant messaging client that can respond to network messages and wake the host when a message is received. The researchers also developed a compact torrent client application that continues to download a file while the host is asleep.
The aim of the USB device would be to reduce the amount of energy that is "wasted" by computers that are left on for trivial tasks for large amounts of time. The device has the ability to perform the low-power intensive tasks on its own, and wake the computer to perform ones that require more processing power. There is also a wireless version available that requires two wireless NICs to function.
The system is only in the prototype phase and still has a lot of testing and further research required before it would be commercially available.