Ever find a cool picture that you really want to make a poster of? I found this site very interesting. Though I haven't tried it personally, in theory, it sounds like it would work.
I can't quite vouch for the amount of ink that you would consume with an inkjet printer to achieve the whole poster on paper, but I would think for a casual poster, it still beats paying the full premium for printing it at kinkos.
You can find the site at http://www.blockposters.com