This plugin uses TCP sockets to send commands to an Arduino over the internet or your local lan. An Arduino is an open source microprocessor development kit. It's cheap and fairly easy to use. The Arduino receives commands either by USB or ethernet, and turns each pin output on or off. These pins can be connected to a relay built into an outlet. Plug a lamp or anything into that outlet and you can control it from a computer program!
ArduinoGC is the program that runs on the Arduino, and can be downloaded here http://downloads.growcontrol.com/ It's an existing project of mine designed for home and green house automation, but it's general purpose enough to work with minecraft too. USB is currently not supported by this plugin, so an ethernet shield is required. A shield is a card that fits on top of an Arduino. Both the WizNET and 28J60 ethernet shields are supported.
I have a test version working, and I'm finishing it up by adding commands and a config.yml