Viking helmets were a historically inaccurate depiction of helmets worn by Vikings, perpetuated by false accounts of Norse mythology. They were metal domes with two horns, one on either side, something which the Tenth Doctor pointed out the Vikings never actually wore.
The touring opera singers aboard Valhalla wore helmets of this design in their performaces of Ultragrossegotterdamerung. Because performaces of this opera were over a month long, the singers' helmets were equipped with technology that could control their actions, making them deliver lines when they had forgotten. (COMIC: Space Vikings!)