Ashnod's Altar

The colorless mana converter that defined a generation of combo design. Before this, sacrifice outlets were either free (Diamond Valley and similar oddities) or attached to a creature; this was the first artifact to convert a creature directly into mana at instant speed, and the two-for-one conversion (one body in, two colorless out) is the engine math that made it a combo piece rather than a value engine. Pair it with any creature that can be cast for less than two mana and replaces itself, and you have arbitrarily large mana; pair it with a recursion loop and you have infinite mana plus infinite sacrifice triggers, which is two combo axes for the price of one. The design has aged into a permanent reference point: every subsequent free sacrifice outlet (Phyrexian Altar, Carrion Feeder, Viscera Seer) is measured against what this one does and what it costs to set up. Wizards has been careful about reprinting it into formats where the loops are too easy, and the card sits on most cube and combo-builder shortlists not because it is splashy but because it is structural: a converter that turns the creature axis into the mana axis without asking for a color, a tap, or a timing restriction beyond the activation itself.