Minecraft wiki entry for Creeper, associated text: 5 experience points, if killed by the player or a tamed wolf.
Between 0 to 2 gunpowders. The maximum amount is increased by one per level of Looting, for a maximum of 5 with a Looting enchantment of level III.
A random music disc (excluding the Pigstep and otherside[1] audiotrack), if killed by a skeleton or stray.
One creeper head, if killed by a charged creeper.