The latest addition to this genre is the Perigallo, designed by Domínguez, García & Ibáñez and manufactured by Spain-based Estudio Sancal.
Rather than nesting or stacking, the Perigallo folds flat for storage.
"The word 'Perigallo' is taken from a local Spanish dialect. It is used to describe a three legged ladder used to harvest local fruit. Domínguez, García & Ibáñez borrowed this traditional idea and have given it a new use. Sancal's Perigallo is a stool that can be folded away when not in use, making it ideal for hosts who suddenly find that they need more seating."
(Note to our Spanish readers: The design team of Domínguez, García & Ibáñez doesn't appear to have a website, at least not that I can find searching in English. If you have any information on them, please do let us know in the comments.)