A level-3 Menger sponge. For a smaller and cheaper model in Frosted Ultra Detail material, check:http://www.shapeways.com/model/245325/menger_frosted.html
Check also the level-4 one: http://www.shapeways.com/model/245336/menger_sponge__level_4_.html
Initially it had reduced size (1mm wall-thickness), until Shapeways said that the model was no cleanable and thus no printable. As they suggested, I have doubled the size of the model, so now it's more expensive. You can find similar models to this from other authors and cheaper because of their reduced size, but surely you'll get a no cleanability error after ordering. The FUD model doesn't have this problems and the holes/walls are 0.3mm wide.