Modular Triangular Bipryamids: 6

These boxes do not have a separate lid and base. Instead, each box is closed by tucking tabs on one unit into pockets on a second unit, then adding a third unit in a similar manner. A box may be opened by reversing this process, or by squeezing it gently at its poles.
 
Richard L. Alexander, the designer of these boxes, has named them Crane's Eggs, even though they are definitely not egg-shaped. The name comes from what he has placed inside each box -- a very small origami crane.