This cardboard Star Wars AT-AT costume is magnificent, and well-documented on a Flickr build-log set. The best Hallowe'en costume I ever had was the cardboard C3PO outfit my kindergarten-teacher mom made out of rolled up tubes of cardboard spray-painted gold — this, however, is miles ahead of what my mom was capable of hacking up. Don't worry Mom, I don't hold it against you.
(Thanks, Gary!)