This year we’re using LibsDisguises, which gives us a broader range of options. Above you can see me using “/d player Dinnerbone setskin PseudoKnight”. You can disguise as almost all entities with a wide variety of options. It’s enabled in all worlds but Frog Park. You can even disguise other mobs! This is especially fun with horses and dogs.

Commands:
“/d <entity> [options]” (disguise yourself)
“/u” (undisguise yourself)
“/dentity <entity> [options]” (disguise another mob)
“/undentity” (undisguise another mob)
“/dhelp [entity]” (get available options for an entity type)

Examples:
“/d irongolem setviewselfdisguise false” (don’t display the irongolem disguise to yourself, so you can go in first person)
“/dentity cow setcustomname Dinnerbone” (set another mob as an upside down cow)
“/dentity player PseudoKnight” (set another mob disguised as me)
“/d horse setbaby true setstyle whitefield setcolor black” (set yourself as a black and white baby horse)

Have a happy Halloween!

UPDATE 2014-11-05: Disguises have now been disabled.