Coil whine! I remember reading that you could use stats on those power fluctuations as a keylogger.
@neftaly maybe we could detect that at a distance using some sort of directed antenna?
@neftaly yes. and the same with the normal EM from the computer. and from the monitor. and the noises from pressing the keyboard. and the reflection of the screen off your eyeball. and the LEDs on your network adapter.