I made a Kalimba from a rake I found in the back lane. All you need is a bunch of metal tines clamped firmly against a resonator, here just a piece of aluminum. The resonator translates the vibrations of the metal tines down into a box, just like the bridge of an acoustic guitar translates the vibrations of its strings into the body. The box is made of thin wood so that it can vibrate freely.
The tines can be tuned by pushing or pulling them.
My favorite thing about this one is that two people can play it at once.
Update: After the photo was taken, I added an aluminum jacket (provided by a tomato can) to the wood bridge, because the tines were eating away at it.