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 by the player. It is not dropped by spiders that died from environmental damage (such as a mob grinder). It can also be eaten but will poison the player, causing 2 hearts of damage.

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

Trivia

 * Although a Spider Eye poisons the player when eaten, it might still be useful for the hunger restoration if there is no other food available, or if you only need to restore 10% or less of your hunger bar (in which case you will soon regenerate the lost health).

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.