The Xandabox (or Raspberry Pi in general) includes a CEC adapter on its HDMI monitor port. CEC is a standard communication protocol supported by most major TV manufacturers. It allows for a playback device (the Xandabox) to power on, power off, and switch video inputs on a CEC-enabled TV. Check out the CEC compatibility chart.
If your monitor does not support CEC (e.g. you have a computer monitor), the Xandabox turns off the HDMI port to allow for the monitor to power-down by itself. This function is supported by all current computer monitors, even some TVs. Kindly take a look here for further details and how to create the script to do that.
If you use a HDMI-to-VGA adapter on the Xandabox, the software will attempt to turn off the HDMI/VGA port, but this might not be supported by this hardware.
In the future, we will provide support for using infrared LEDs to switch the monitor on or off through IR, just like normal TV remotes do. For large installations, we can support custom solutions, like the above, or custom serial connections to monitors. Also, we will be providing programming hooks that will allow you to create custom scripts to power the monitor on/off.