/setblock command

setblock <x> <y> <z> <tilename> [datavalue] [oldblockHandling] [dataTag]

► Create an enchanting table at a particular location regardless of what block was in this location previously.

/setblock -251 68 254 minecraft:enchanting_table

► Create an apple spawner that spawns a new apple every 10 seconds. 1 in 5 apples are a golden

apple and they despawn after 3 seconds.

/setblock -241 72 259 minecraft:mob_spawner 0 replace {EntityId:Item,SpawnData:{Age:5940,Item:{id:minecraft:apple,Count:1}},SpawnPotentials:[{Type:Item,Weight:5,Properties:{Age:5940,Item:{id:minecraft:apple,Count:1}}},Type:Item,Weight:1,Properties:{Age:5940,Item:{id:minecraft:golden_apple,Count:1}}}],SpawnCount:1,SpawnRange:7,RequiredPlayerRange:10,MinSpawnDelay:1,MaxSpawnDelay:10,MaxNearbyEntities:10}

► Replace the block beneath a horse with a hay block.

/execute @e[type=EntityHorse] ~ ~ ~ setblock ~ ~-1 ~ minecraft:hay_block

► Place layers of snow if a thrown snowball is found within a particular area.

/execute @e[type=Snowball,x=-258,y=58,z=250,r=15] ~ ~ ~ setblock ~ ~ ~ minecraft:snow_layer 3 replace

Notes:
Values for oldBlockHandling include replace, keep or destroy. Replace is the default value if oldBlockHandling isn't defined.

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