Stewart Coffin's Peanut Puzzle, modified by starting from 3cm diameter spheres. Includes six different pieces (the Coffin puzzle) plus two "end caps". The standard puzzle assembles into many different shapes, with the end caps literally thousands of shapes become possible.
The pieces will fit tightly out of the box. I recommend scrubbing them with a toothbrush in soapy water for a perfect fit. One nice finishing technique is to dye them 7 different colors.