metapixel photomosaic   (link)

http://www.complang.tuwien.ac.at/~schani/metapixel/
Metapixel is a program for generating photomosaics. It can generate classical photomosaics, in which the source image is viewed as a matrix of equally sized rectangles for each of which a matching image is substitued, as well as collage-style photomosaics, in which rectangular parts of the source image at arbitrary positions (i.e. not aligned to a matrix) are substituted by matching images.