A fun box is a 3 by 3 structure made entirely out of hostile mob spawners. Vechs likes to place them throughout his maps to provide a challenge to the player. Many funboxes are also encased in smooth stone, making them more easily to distinguish from the surrounding terrain (and providing a small amount of cobblestone which can be useful on maps which limit the amount of stone available to the player).
Be warned, when finding one it is best to destroy it from an area where the mobs can't reach you. A funbox can spawn more mobs than a player can kill, which makes it impossible to disable by rushing in and placing torches.