In a charming example of wordplay, Italian photography student Francesco Capponi took the Italian word "pinolo" (pine-nut) and turned it into Pinholo, the pine-nut pinhole camera. I could be wrong, but this could be the world's smallest pinhole camera. Even smaller than this dinky little camera, and most definitely smaller than Capponi's previous project, which was an egg pinhole camera.
Capponi crafted the pinhole camera by painting the inside of the pine-nut shell black, and drilled a tiny hole in one of the sides. After squeezing a little piece of photographic paper into the shell, two metal rings were affixed to secure the two halves of the nut shell together to keep out the light. As the pinhole camera was too small to add a shutter release, Capponi just used his thumb to "take" the photo by removing it from the hole, and training a flash light on his face, or in the example above, his hand.
The photos needed blowing up a bit after being scanned into his computer (I shudder to think of the eyesight you'd need to see pine-nut sized photos), but you can see a few more b&w examples over here. [Lomography]