If your looking for a small pin hole camera then this mini Lomo Camera by Francesco Capponi has to be the smallest.
This tiny pin hole camera actually takes photos and has to be the smallest I have ever seen, small enought o fit on the tip of your finger.
Creator Francesco Capponi uploaded photos of his mini lomo camera creation on to MAKE’s photo pool on Flickr. You can check photos take with this tiny camera on Francesco Capponi Flickr site
Unfortunately there are no details on how Capponi made this little tiny camera but from the photos it looks like a wood and card construction.