Urza's Incubator

Most cost reduction keys off a card type or a color; this one keys off a creature type, named the moment the artifact arrives, which makes it color-agnostic in a way that genuinely matters. A Goblins deck, an Elves deck, a Dragons deck, and a Slivers deck all read the same discount off the same three-mana artifact. The design lesson baked in is that the cut lands on the spell, not the permanent, so it compounds with every creature you cast and rewards committing hard to one tribe rather than splashing across two. The choice happens as the artifact enters, not on resolution, which is a sharper distinction than it looks: even an artifact put onto the battlefield without being cast still names its type the moment it arrives, and that type is locked in for good. Commit to a single creature type and the card snowballs; hedge across two and it does nothing for half your deck. It also predates the keyword-counter and changeling tooling that later let tribal decks blur their lines, so its discount comes cleaner than the modern designs that try to do the same work with more conditions attached. Among the early artifacts built to subsidize a creature type, this is the most frictionless expression of the idea: no upkeep cost, no triggered tax, just a flat cut that keeps paying out every time you cast a creature of the chosen tribe.