This is a very easy and fun to build project that will allow you to control up to eight external devices through your computer's parallel port. You may for instance control different appliances such as lamps, computers, printers, tv sets, radios, music systems, air conditioners, air fans, garden sprinklers and anything else you can think of, all through your computer.
You can do a simple test by connecting a LED directly to a ground and any PINs 2 through 9 of your parallel port. The voltage on the parallel connectors is around 4.5V but it drops to around 2V once you connect the LED. You may use a 100 Ohm resistor if you want to, but it is not necessary since the current is very low.