Actors who stripped down: Too many to mention
Why they're fully nude: Because the fascist powers that be said so.

Still banned in certain corners of the world today, when it comes to daring to plumb the deepest, darkest places of the human psyche even The Human Centipede movies (obvious descendants) have got nothing on Salò. Based on the Marquis de Sade's book The 120 Days of Sodom, Salò uses nudity as a tool for exploring larger ideas of corruption, perversion, and fascism.

Directed by Pier Paolo Pasolini, the film tells the story of four men of power—The Duke, The Bishop, The Magistrate, and The President—who kidnap 18 young men and women, whom they then subject to various forms of physical, mental, and sexual torture (with four middle-aged prostitutes helping to devise new and even more humiliating ways to up the ante). Fecal consumption, scalping, and branding are just a few of the tactics that are used. 

Though it can't technically be classified as a horror film, that didn't stop the Chicago Film Critics Association from giving it the number 65 spot on their Top 100 Scariest Movies list. The film's unrelenting violence and wall-to-wall full-frontal nudity are as effective a statement on power and corruption today as they were upon the film's release more than 35 years ago. Salò still shocks.