Anyone who played Minecraft probably knows about this circuit, because it's pretty popular and also not that hard to make. The first person who recreated this circuit in SC was laurensweyn.
If someone doesn't know what it does - it has three inputs (switches ) and two outputs (lighbulbs ). When you switch any switch, lightbulb marked "1" will turn on. When you switch any two switches, lightbulb "2" will turn on and the "1" will turn off. And finaly, when you switch all the three switches, both lightbulbs will turn on (because 1+2=3).
Although it's easy to build (with the help of pictures), I've decided to put a link to the world file here.
Although this circuit can be used in computers and so, most people use it to make their friends be impressed by it.