/blockdata command

blockdata <x> <y> <z> <dataTag>

► Change the text of a sign relative to the command execution location.

/blockdata ~ ~1 ~ {Text2:Commands are,Text3:fun!}

► Change pattern for a banner at a particular location.

/blockdata -254 74 259 {Base:0,Patterns:[{Pattern:"bri",Color:1},{Pattern:"cre",Color:15}]}
/blockdata -254 74 259 {Base:0,Patterns:[{Pattern:"gra",Color:12},{Pattern:"sku",Color:15}]}

► Change the primary beacon power to haste for a beacon at a particular location.

/blockdata -257 69 258 {Primary:3}

► Set cake, cookies and milk items within a chest (trapped or regular, not ender).

/blockdata ~ ~1 ~ {Items:[{id:minecraft:cake,Slot:12,Count:1},{id:minecraft:cookie,Slot:13,Count:3},{id:minecraft:milk_bucket,Slot:14,Count:1}]}

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