Yesterday, when digging around the bins at a Goodwill salvage store, I found a strange brick. It seems to be two pices, a white 1x3x3 base and a black 3x3x3 top with a triple-sized stud. The stud has four air holes in it, and a big LEGO logo. The parts can be separated by a 30-degree twist, and the white part doesn't itself have LEGO stamped in it, so presumably both pieces came together in pairs. The white part just perfectly grips onto 3x3 regular Lego studs, but not Mega Blocks, and the black part has thinner walls at its base, so it won't grip at all.
I can vaguely recall seeing one of these before, and I also have a vague idea that they were for some kind of kids meal promotion and came with a few small bricks inside.
Trying to search for info has turned up nothing, probably because I don't know what its correct name is or where it came from.