The foam kinda gives it away, but you probably wouldn't otherwise be able to tell that these SNES cartridges are actually soap bars and only effective in the bathroom. They're available on Firebox as various classic titles, such as Super Mario Kart, Donkey Kong Country or Street Fighter II Turbo.
Too bad they haven't made soap bars posing as rare SNES titles, just for the fun of it.
There are Game Boy versions, too. They're also pretty cool:
Super Nintendo Gamer Soap Cartridges [Firebox, via The Laughing Squid]
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