d12 is the loneliest die. It is used for barbarian hit dice and greataxe damage in Dungeons & Dragons 3e.
The fact that Orcs (and Half-Orcs) are both the most common barbarians and the most common wielders of greatatxes, it is suspected that Gruumsh is the head of a conspiracy aiming to eliminate the d10 in favor of the d12, in order to "purify" dice sets so that they consist only of true platonic solids.
Observant smar/tg/uys will notice that d12 are the hitdice used for undead and dragons instead of the usual d8s.
d12 is the highest stats can ordinarily go in Savage Worlds. BBEGs and DMPC Mary Sues can have d20, but it's hella rare.
d12 shows up in Cthulhu-themed games, like Pokethulhu or Cthulhu Dice, probably because d12 is the mutant freak of dice. It's also used quite a bit in Ironclaw, because furries like to imagine that they're abused or neglected so they take neglected dice.
One of the few places where the d12 is not lonely, and is in fact used a great deal, is the DragonMech campaign setting. Its chief use there is for damage rolls with mech weaponry.
Try and make ME go outside? Fuck off, d12.