Hentai Games (R=18)

No eroge novels (aka 'erotic visual novels') in this list: there are so many of them that we would need another page just for them.

ZONE games aren't really games, but sweet Jesus does 4chan love them and each one is well-made. They are in a separate list below.

DOS games
You'll need an actual emulator like DOSbox for these; running it in windows will just get you sadface.gif. We don't mention whether they are free because the software would be so old it's unlikely anyone's gonna enforce it unless you're reselling the stuff.

Flash games
The flash games aren't windows-dependent, so they don't have stupid stuff like region checking, or needing an emulator. If there isn't a link in the description, try swfchan.com.

ZONE games
All ZONE games are in english, are flash and they're NOT free (you have demos though), you can get them at his/her website.