A few random potions for a gray and windy day. I like potions because they're easily managed packets of power that can be exceptionally powerful without being unbalancing. After all, once they're used, they're gone for good.
Silverskin - A shiny, silvery liquid with a sharp smell and extremely bitter taste. When consumed it causes the imbiber's skin to take on the color and shine of a polished silver mirror. All elemental spells directed at the imbiber are reflected back at the caster for full effect.
Aqueous Form - A clear, viscous liquid that is both odorless and tasteless. When consumed the imbiber's body transforms into coherent water. They can retain their shape, or flow through small openings or crevices at will. The imbiber could, for example, put out a torch simply by dousing it with their hand, or drown someone by smothering them with their body. While under the effects of this potion the imbiber is immune to mundane weapons and most elemental attacks, though cold damage is increased by two ranks.
Winter's Breath - Each dose of this pale blue and minty potion allows the imbiber to breathe forth a gust of bitterly cold air, inflicting Great damage on anyone within the area of effect, a 20 foot long, 5 foot wide path. Any water in this area of effect will be frozen solid, making this potion useful for creating temporary bridges or dams.
And now for my favorite potion story: In a D&D campaign I ran a few years back, Father Bo, the party's cleric, got saddled with a potion of sweet water. The player grumbled and stowed it away, not really thinking much of the item. Weeks later the party was investigating events in the sewers beneath a coastal city. The sewers were routinely flooded by the ocean's high tides and one of the pools had been occupied by a gigantic crevice creature. The beast surprised the party as they were crossing the pool on a narrow walkway, and quickly dragged several party members into the water. Good old Father Bo pulls out his sweet water potion, uncorks it, and tosses it in the pool, instantly transforming the salt water to fresh and killing the crevice creature in the process due to osmotic shock. Never let it be said sweet water is a useless potion!