The Za, from Misosoupdesign.com is a stackable stool with several possibilities. The basic shape is a free standing unit with some strategically placed slots. Use it alone to sit on the floor, Japanese style, if you need a backrest. Several of these units can also stack up neatly forming low slung chairs. The stacked configuration naturally also saves space if storage is an issue.
However, the clever bit is when two of these units are interlocked by sliding them together via the slits. Then they form a stool or even a small table. Neato!!