List of valid entities. Taken from bukkit.org
For an updated and compelte list, see http://jd.bukkit.org/doxygen/d5/d27/namespaceorg_1_1bukkit_1_1entity.html

Animals
    Chicken			Represents a Chicken.
    Cow				Represents a Cow.
    MushroomCow     Represents a mushroom Cow.
    Ocelot        	A wild tameable cat.
    Pig        		Represents a Pig.
    Sheep        	Represents a Sheep.
    Squid        	Represents a Squid.
    Wolf        	Represents a Wolf.

Monsters
    Blaze			Represents a Blaze monster.
    CaveSpider		Represents a Spider.
    Creeper        	Represents a Creeper.
    EnderDragon     Represents an Ender Dragon.
    Enderman        Represents an Enderman.
    Ghast        	Represents a Ghast.
    Giant        	Represents a Giant.
    MagmaCube       Represents a MagmaCube.
    PigZombie       Represents a Pig Zombie.
    Silverfish      Represents a Silverfish.
    Skeleton        Represents a Skeleton.
    Slime        	Represents a Slime.
    Spider        	Represents a Spider.
    Zombie        	Represents a Zombie.

Denizens
    IronGolem       An iron Golem that protects Villages.
    Snowman        	Represents a snowman entity.
    Villager        Represents a villager NPC.

Projectiles
    Arrow			Represents an arrow.
    Egg        		Represents a thrown egg.
    EnderPearl      Represents a thrown Ender Pearl entity.
    Fireball        Represents a Fireball.
    SmallFireball   Represents a small Fireball.
    Snowball        Represents a snowball.
    ThrownExpBottle Represents a thrown Experience bottle.
    ThrownPotion    Represents a thrown potion bottle.

Environment
    EnderSignal     Represents an Ender Signal, which is often created upon throwing an ender eye.
    Explosive       A representation of an explosive entity.
    FallingSand     Represents a falling block.
    TNTPrimed       Represents a Primed TNT.

Ungrouped
    Boat			Represents a boat entity.
    EnderCrystal    A crystal that heals nearby EnderDragons.
    ExperienceOrb   Represents an Experience Orb.
    Fish       		Represents a fishing hook.
    Item        	Represents an Item.
    Minecart        Represents a minecart entity.
    Painting        Represents a Painting.
    Player        	Represents a player, connected or not.
    PoweredMinecart Represents a powered minecart.
    StorageMinecart Represents a storage minecart.
