It's not often that adding a shiny new electronic device to your set-up can reduce electricity usage, but Josh Forwood's Power-On-PC project is designed to do just that. Recently brought to our attention by Hackaday, the device itself is a self-build based around the Arduino Mega which allows the various computers on Josh's network to be placed into an almost-but-not-quite off mode where the network interface is still powered up. When Josh - or any other user with a PC on his network - logs into a Teleduino-based web interface they can use the Arduino to send a magic packet which is detected by the system's Wake-on-Lan capability, powering it up just as though someone had pressed the button. The result: the systems can be left powered down until they are needed, saving money and the environment to boot.
Hardware wise, this couldn't be simpler, and involves an Arduino running as a server. The Arduino can be plugged anywhere into my home network and long story short, allows me to send a WOL magic packet from outside my internal network.