These are great organizers! And, yes, they stack - they have handle openings on either narrow end - one juts inward and the other outward. To stack, just stack them opposite of the one below them - so the bottom one has the end that juts outward by the open end of your self, the one on top of it needs to have the end that juts inward toward the open end. Alternating allows them to stack, having them all the same direction means they nest for storage.
Ok, so on to the other important bits - these are a nice sturdy, thick clear plastic. They do not flex or bend and they bear weight well, even when stacked. They're easily wiped clean with a damp cloth and I've hand washed mine in hot water when things were spilled inside and they look just like new.
I use these in the pantry and fridge but have not used them in the freezer so I cannot attest to their durability when used there. I have dropped these onto the floor at one time or another, both straight from the fridge or off of the pantry shelf, with and without weight inside them and they've never cracked (which is a really great, accidental, test of their durability.)
I own a couple sets of these from different brands and this set, this brand, is my favorite, even though it was not the most expensive or best reviewed. I only wish it were also possible to buy more of these in separate sizes - like a set of 4 large ones or 4 small ones, without buying the mixed set.