/summon command

summon <EntityName> [x] [y] [z] [dataTag]

► Spawn a charged creeper.

/summon Creeper ~ ~ ~ {powered:1}

► Spawn a Enderman named Bob carrying a command block.

/summon Enderman ~ ~ ~ {carried:minecraft:command_block,CustomNameVisible:1,CustomName:"Bob"}

► Spawn a baby zombie holding a iron sword riding a skeleton horse which is also ridable for players (change EntityHorse type to 3 for a dead horse).

/summon Zombie ~ ~4 ~ {IsBaby:1,IsVillager:1,Equipment:[{id:minecraft:iron_sword},{id:minecraft:iron_boots},{id:minecraft:iron_leggings},{id:minecraft:iron_chestplate},{id:minecraft:iron_helmet}],Riding:{id:EntityHorse,Type:4,Tame:1,SaddleItem:{id:minecraft:saddle}}}

► Spawn a Giant holding a sharpness 3 gold sword and wearing protection 1 chain armour.

/summon Giant ~ ~2 ~ {Equipment:[{id:minecraft:golden_sword,tag:{ench:[{id:16,lvl:3}]}},{id:minecraft:chainmail_boots,tag:{ench:[{id:0,lvl:3}]}},{id:minecraft:chainmail_leggings,tag:{ench:[{id:0,lvl:3}]}},{id:minecraft:chainmail_chestplate,tag:{ench:[{id:0,lvl:3}]}},{id:minecraft:chainmail_helmet,tag:{ench:[{id:0,lvl:3}]}}]}

► Spawn multiple animals at once.

/summon Bat ~ ~2 ~ {
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:Sheep,CustomName:"jeb_",CustomNameVisible:0,
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:SnowMan,
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:Wolf,
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:Chicken,
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:Cow,
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:MushroomCow,
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:Ozelot,CatType:1,
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:Rabbit,RabbitType:5,
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:Pig,Saddle:1,
Riding:{id:Item,Age:5980,Item:{id:minecraft:string},
Riding:{id:EntityHorse,Type:3,Tame:1,SaddleItem:{id:minecraft:saddle}
}}}}}}}}}}}}}}}}}}}}}

► Spawn a diamond chestplate with protection 5 and unbreaking 3 and the chestplate won't despawn or can be picked up.

/summon Item ~ ~1 ~ {Item:{id:minecraft:diamond_chestplate,Count:1,tag:{ench:[{id:0,lvl:5},{id:34,lvl:3}]}},PickupDelay:32767,Age:-32768,Health:32768}

► Spawn a iron chestplate with protection 5 and unbreaking 3 and the chestplate won't despawn or can be picked up within the first 20 seconds of spawning.

/summon Item ~ ~1 ~ {Item:{id:minecraft:iron_chestplate,Count:1,tag:{ench:[{id:0,lvl:5},{id:34,lvl:3}]}},PickupDelay:400,Age:-32768,Health:32768}

► Spawn a golden chestplate with protection 5 and unbreaking 3 and the chestplate won't despawn but can only be picked up by a player called Steve.

/summon Item ~ ~1 ~ {Item:{id:minecraft:golden_chestplate,Count:1,tag:{ench:[{id:0,lvl:5},{id:34,lvl:3}]}},Age:-32768,Health:32768,Owner:"Steve"}

► Spawn a bottle of super juice, a custom potion of regeneration item drop

/summon Item ~ ~4 ~ {Item:{id:373,Damage:8193,Count:1,tag:{CustomPotionEffects:[{Id:10,Amplifier:2,Duration:3000}],display:{Name:"Super Juice"}}}}

► Spawn a jump boost splash potion riding an xp orb

/summon ThrownPotion ~ ~8 ~ {Potion:{id:373,Damage:16395,Count:1,tag:{CustomPotionEffects:[{Id:8,Amplifier:2,Duration:200}]}},Riding:{id:XPOrb}}

► Spawn a falling chest containing a sharpness 5 diamond sword and apples

/summon FallingSand ~2 ~20 ~ {Block:minecraft:chest,Time:1,TileEntityData:{Items:[{id:minecraft:diamond_sword,Slot:13,tag:{ench:[{id:16,lvl:5}]},tag:{display:{Name:"My Little Friend"}},Count:1},{id:minecraft:apple,Slot:0,Count:1},{id:minecraft:apple,Slot:2,Count:1},{id:minecraft:apple,Slot:6,Count:1},{id:minecraft:apple,Slot:8,Count:1},{id:minecraft:apple,Slot:10,Count:1},{id:minecraft:apple,Slot:16,Count:1},{id:minecraft:apple,Slot:18,Count:1},{id:minecraft:apple,Slot:20,Count:1},{id:minecraft:apple,Slot:24,Count:1},{id:minecraft:apple,Slot:26,Count:1}]}}

► Exploding Ghast

/summon PrimedTnt ~ ~20 ~ {Fuse:55,Riding:Ghast}}

► Spawn a rocket exploding TNT

/summon PrimedTnt ~ ~1 ~ {Fuse:15,Riding:{id:FireworksRocketEntity,Motion:[0.0,0.4,0.0],LifeTime:15,FireworksItem:{id:minecraft:fireworks,Count:1,tag:{Fireworks:{Explosions:[{Flicker:1,Trail:1,Type:1,Colors:[16711680],FadeColors:[1499900]}]}}}}}

► Spawn a deadly fireball

/summon Fireball ~ ~3 ~ {Motion:[0.0,0.0,0.0],direction:[0.0,0.0,0.0],ExplosionPower:50}

► Make lightning strike all nearby villagers

/execute @e[type=Villager] ~ ~ ~ summon LightningBolt

Notes:
Equipment is defined with Equipment:[{id},{id},..] where the slot order is item held, boots, leggings/pants, chest and then helmet. Enchantments can also be defined per equipment id, for example: {id:minecraft:golden_sword,tag:{ench:[{id:16,lvl:3}]}} which defines a golden sword with sharpness 3.

More information:
http://minecraft.gamepedia.com/Data_Values
http://minecraft.gamepedia.com/Chunk_format
http://minecraft.gamepedia.com/Commands