A single ski. It is a Special Item, in that it can appear only once per run at most, is priced at a higher price in shops, and is overall harder to find due to increased rarity.
The ski allows its holder to travel faster when moving over unexplored ground. It snows during level generation, bringing forth fresh powder for you to make use of. Travelling over any new ground will boost your speed, but by going over you use up the snow and spots you have gone over lose the benefit.
NPCs such as lesser and greater monsters will not affect your precious powder, as they are unconcerned with frozen precipitation.