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 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

 * 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.

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.