Spider Eye

The Spider Eye was first added in the Beta 1.9 Pre-Release 2. It is dropped by either Spiders or Cave Spiders when killed. It can also be eaten but will poison the player, causing 3 hearts of damage.

Crafting
Spider Eye can be combined with Brown Mushroom and Sugar to craft Fermented Spider Eye.

Trivia

 * because spider eyes are only used to make poisonous potion or poisons the player it's a quite useless item

Potions
As of Minecraft 1.9 Prerelease 3, if a Spider Eye is mixed into a Awkward Potion it will create a Poison Potion. A Poison Potion hurts the player for a set amount of time, just like getting attacked by a Cave Spider.