A traditional folk/blues instrument, by far one of the cheapest I've ever encountered.

To make one, put two nails in a wall that you don't mind having a pair of holes in. Next, stretch a length of wire (traditionally from a broom though piano wire is said to work better) between the nails, making sure it's tight. If you need, place something under the wire that's a bit taller than the length of nail that is sticking out of the wall for more tension. There, you have a diddley bow. To play it, just pluck the wire while running a knife, nail, bottle or a piece of wood along the string to change the pitch.

If you want to hear a diddley bow being played, the only cd I've heard with one on it is Scott Ainslie's Terraplane.

Incidentally, it's said that Bo Diddley took his name from this instrument.