Plug computers are tiny computers mostly running Linux operating system. Features and components vary from device to device. Some of the plug computers have HDMI ports, USB ports, Ethernet ports, Wifi and expansion card slots. You can connect a portable hard disk to one of the USB ports and use the device as a DLNA server which streams audio and video to supported devices.
There are hundreds of uses including Security surveillance, web hosting, Media streaming etc.
A good practical use would be developing and deploying an IM bot which executes certain tasks such as updating your Facebook status, Sending an SMS etc, when you message the IM bot. Cool huh! All you need to do is command the bot through your favorite chat application.
What about running Smoothwall on the device? Smoothwall is a powerful open source firewall for the Linux OS. If your router's built-in firewall isn't enough for you, then Smoothwall is the answer.
In fact some of these devices are a development platform, on which you can deploy various applications. Its definitely a must have gadget for the geeky Geeks. Enjoy!