Sicherman dice are a variant of traditional 6-sided dice (aka the d6), that were developed by George Sicherman in 1977. They are designed to give the same results as two d6 summed together (2d6), but have an different set of pips than traditional d6's.
Traditional dice have sides numbered 1, 2, 3, 4, 5, and 6. When you roll two d6's, the probability distribution makes some numbers quite rare (2 and 12, for example), and others quite common (e.g., 7). Sicherman dice are numbered 1, 3, 4, 5, 6, 8 on one die, and 1, 2, 2, 3, 3, 4 on the other. When rolled as a pair, these produce exactly the same probability distribution as the traditional pair of dice.
Obviously, this only works if you are rolling a matched pair of dice. Less obviously, this is the only other set of whole, positive integers that will produce this probability distribution on any set of two six-sided dice.
They were initially brought to public attention by puzzlesmith Martin Gardner in a 1978 Scientific American article. These dice are only useful as a mathematical curiosity, and have no need to exist as physical objects. Needless to say, they have since been produced by a number of weirdos around the world.