The Mad Scientist Extension Cord uses a monster movie style knife switch to turn power to the extension cord on and off. I like using it to turn my Jacob's Ladder on and off. This whole setup consists of a heavy duty 3PDT knife switch and 18 AWG 3 conductor fabric wire that is rated for up to 5A of current.
The high voltage might make this seem complicated, but it is a rather simple project. In fact, we are only wiring up half of the switch's connections, such that is wired more like a 3PST switch. Ultimately, it is just an extension cord with a switch in the middle. We are simply extending the three prongs from the wall socket and making or breaking each of three connections.
While this is simple, I'd once again like to point out that the electrical contact from the wall is slightly exposed, and there exists a real chance of electric shock if used wrong. You will need to be careful when handling this, and it is not good to leave plugged in anywhere you may have children, small animals or people who don't understand electricity. Since I have just described most places, it is not recommended you keep this plugged in when not actively demonstrating it. In fact, is likely in violation of most building codes in most places, so this is only a novelty item that you should always be careful when handling, and never leave plugged in or unattended. But it's great for your Halloween Haunted House display, or for impressing house guests.
To learn more about electricity and switches, check out the Electronics Class.