It's not exacly hentai, but for me they're somehow equivalent. Regrettably, most of them are in Japanese and can get quite difficult to navigate.
I'll add some of my links here; if you're interested, why not contributing with some more?
(Warning: adult only)
extremely good style. Very evoking.
large collection, good as introduction. Not hard enough for my tastes
several stories for all tastes. Some of them very good.