Magma Armor

Introduced in Generation III, Magma Armor provides complete immunity to the frozen status condition. The Pokemon cannot be frozen by any means -- whether through moves with a freeze chance like Ice Beam and Blizzard, or through the rare Tri Attack secondary effect. If the Pokemon is somehow frozen and then acquires Magma Armor through Skill Swap, the freeze is immediately thawed.

Outside of battle, Magma Armor has a highly practical utility effect: if any Pokemon in the party has this ability, all Eggs in the party have their hatch cycles reduced by half, significantly accelerating the breeding process. This makes Magma Armor Pokemon staples in any serious breeding operation. Prior to Generation IV, the overworld egg-hatching effect did not exist.

Found on only 3 Pokemon -- Slugma, Magcargo, and Camerupt -- all Fire-types whose molten bodies logically prevent freezing. While the in-battle freeze immunity is situationally useful, the egg-hatching utility is by far the most commonly exploited aspect of this ability.