This is a Minecraft mod, originally from November 2012. Based on copygirl's external tool NBTEdit, In-Game NBTEdit allows users to edit NBT tags of entities and tiles while in-game.

The code is fairly messy - I would recommend porting this to GuiLib, but that is easier said than done.

Forum Thread: http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/1286750-in-game-nbtedit-edit-mob-spawners-attributes-in