World object

World objects are items that appear in the game's environment but cannot be taken by the player character into their inventory.

Wooden crates
For example, there are two of these wooden crates found in the Megaton House. Note that in at least one confirmed case on the 360, these crates respawned back at their original locations sometime after having been moved.

Metal Barrels
Those large, dented, tan metal barrels found just about anywhere. Not that these can be used to covertly kill NPCs with their physics damage, although it is a painstaking process.

Shopping Carts
Several can be found in Paradise Falls, for instance. They make a distinctive, loud noise when kicked or pushed.