A electronic device used to tap into phone lines. It is extremely easy to make and use one: Take a phone, cut the phone jack off the end, strip the red and green wires back a few centimeters and attach alligator clips. Voila, you've made a beige box.
Now the second and harder step is finding a place to hook those alligator clips up to. You may have noticed those brown/green lengthy boxes sticking out of the ground usually around houses or places of business. It's actually kind of hard to miss them as they are everywhere. The coverings of these (usually held on by two large screws) can be removed with a pair of needle nose pliers and about five minutes of work. Once inside you'll find a massive collection of wires. Hook up the proper two wire combination to your beige box and you'll be rewarded with a dial tone.
See beige boxing for more details.