The First 15 Minutes of Thrill Kill (PlayStation)

Copied from Infogalactic:

Thrill Kill is an unreleased 1998 fighting video game for the Sony PlayStation. While the technical feat of allowing four players to fight simultaneously in the same room was to be a major selling point, this was overshadowed by the controversy surrounding the game's depictions of violence and sexual content. Examples of this content include BDSM and fetishistic costumes and acts, limb dismemberment, and violent special moves with names such as "Bitch Slap" and "Swallow This".

As it was essentially completed before it was cancelled and its code leaked onto the internet, it is one of the most widely available and easily playable unreleased games ever made. Discs and downloads containing near-final versions of the game are easy to obtain via the internet and are playable on PlayStation emulators and modded consoles.

The game is notable for being the first video game to get an extremely rare AO rating for its violence, which would later be followed by the PC release of Manhunt 2 in 2009, and Hatred in 2015.