Plug computing is really coming of age, and in my opinion - that's very cool!
A plug computer is often described as a small, low-power computer that has a form factor that allows it to be plugged directly into a power outlet.
They've been around for awhile as specialty hardware. I think the first one I saw was called JackPC, and that was designed more as a computer that resembled a wall outlet than something that would get plugged into a wall outlet.
More recent plug computers look almost like power adapters, aka wall warts. The one that seems to have gathered the most attention is called the SheevaPlug. It runs on the ARM9 architecture and is marketed by Marvell and their system-on-chip called Kirkwood 6281.
I've read that the SheevaPlug only uses 5 watts of power. If so, that would really be amazing because its reportedly quite a little powerhouse, too, in terms of processing power.
I'd first guess that a very useful application for a plug computer would be wireless - using the plug as a node in a wireless mesh network, or simply acting as a repeater node to extend wireless range.
After that, the sky is the limit! Prior to plug computers, enthusiasts have been modifying wireless routers with linux to act as micro-servers, even using some that have support for USB as network attached storage devices.
This brings up two important points about some plug computers:
The eSata support is really, really awesome!
This is another important factor when it comes to plug computers: they are inexpensive! The SheevaPlug development kit can be purchased for only $99 USD!