The humble LED which stands for Light Emitting Diode. As a diode it means that electricity can only flow one way. There are two legs (pins or wires), a short one and a long one. The long one is the positive and the short one is the negative.
You can get them in a variety of sizes and colours. Up until recently they were only found as indicators on electronic devices usually to tell you if it is on, off or on standby. Now they are used as the main light bulb in house and headlamps on cars.
There beauty is in it simplicity. It is very versatile and easy to control. It also uses very little power and most of the energy is converted to light rather than heat (as with a convention bulb).