i love ideas that create spaces without a footprint. by far, the best example is this martha creation. it's a totally functional, suspended shelf that even creates a wall vignette, but takes up no precious walking room in a space of any size.
another idea i think is pretty smart in theory is this interesting foldout desk from world market, although it's sort of astonishingly ugly when closed. i would have to do something to the outside, otherwise i would end up just leaving it unfolded all the time, which would sort of defeat the purpose. but maybe painting it with a high-gloss paint so it looks laquered would help, and then maybe adding a lightweight framed picture or piece of art would help even more.