Spider Eye

Spider Eye was a food item added in Beta 1.9 Pre-Release 2. It is dropped by either Spiders or Cave Spiders when killed by the player, but is not dropped by spiders that died from environmental damage (such as a mob grinder). It can be eaten, but will poison the player for four seconds, managing to cause of damage.

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

Trivia

 * Despite the fact that the Spider Eye is poisonous and can be consumed to slightly replenish hunger, it will only harm a player's health instead of just the player's hunger.
 * It could even be used when one's health bar is down to a few hearts, because poison cannot kill player (it reduces the health to at most).
 * Although Spider Eyes can only produce negative affects for potions, they can be used as an attack when made into a Splash Potion (brewing Gunpowder into a potion will make it a Splash Potion).

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.