Jake of the Web

http://www.blockposters.com/
Will take any image under 1 Mb and blow it up to poster size for you, with the poster consisting of 8 1/2 X 11 sized 'blocks'. It generates a PDF file with as many pages as necessary, which you can then print them out on your own and tape together. You can save it in digital format and print it out as many times as you like. You can click on the 'gallery' link at the top to see what people have used the site for. One kid used it to make his own cardboard cutout of a stormtrooper.