This has been one of my pet loves for a while because of the Non Non Biyori mod I have.
Left 4 Dead 2 can become ridiculously lewd with a few mods. See: http://imgur.com/Xnll3Ex

A collection of mods makes the game into completely strange and lore breaking anime apocalypse nightmares.
It features Neru, Teto, Miku and Haku, the Vocaloid characters as the 4 L4D2 character-people and you can run around as a pretty anime girl.

Click here.


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If you just specifically want the Non Non Biyori mod, it takes around 15-20 minutes work and you need to edit L4D2 archives.
It's worth it though, and you get the Non Non Biyori Repeat OP as your background video and the music, of course, plays too.

Click here.

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Example and extended screenshots of modding L4D2: http://imgur.com/Xnll3Ex