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.

Though more information is needed it seems in the 1.9 pre-release version 2 spiders that are killed "naturally" or at least by cactus do not drop spider eyes. This maybe intended or be a bug. Further research is necessary to see if this drop is caused by both mob to mob interaction or just player to mob interaction. This is confirmed to be used in potions.

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