Broadhurst Architects, Inc. have designed a prefab shelter called "The Crib," that seems great for an outdoor weekend retreat, or as a home addition. With a great utilization of space, and a creative use of a garage door style opening, I'm definitely a fan.
http://www.thecrib.info/full.htm via Jetson Green
The tiny structure is open to the elements (other than optional screen panels and doors) and can be assembled in a few days.
Green features, some optional and others not, include LED and CFL lighting, efficient heating and cooling systems, rainwater recapture, and construction with recyclable materials.
http://www.thecrib.info/full.htm via Jetson Green