Plunger

is an item found in Fallout 3 and Fallout Tactics. In Fallout 3 it is useful only as Rock-It Launcher ammunition, while in Fallout Tactics it can be used as a weapon. Located in toolboxes, toilets and storage rooms across the wasteland.

Fallout 3
Inside the DC ruins hides a room called the Plunger Room of Death. This room is located inside the Shelter and holds multiple plungers, a single Feral Ghoul, and bloody hand prints decorating the walls and ceiling. Video

Fallout Tactics
Plungers can be used as melee weapons, but are not especially effective.