Michael A Salter's 'Styrobots' are constructed from discarded polystyrene packing materials. The biggest styrobot he has created, 'Giant Styrobot', stood at 22 feet tall: I love how Salter uses recycled materials to create such sleek, white forms. The textures and lines/patterns here are all very inspiring... ('Giant Styrobot' from Robots: A cultural Icon exhibition at San Jose Museum of Art, California 2008)
I've always thought that yarn-bombing was an especially interesting form of street art. These knitted telephone boxes are so sweet and surreal. You can read about the covers (and the Yarn Corp's ensuing brush with the fuzz) at Knit the City.