Used to be backpage and Craigslist I think. Then that got shutdown by the federales. Now I think the big one is leolist. You wouldn't think it would work out in the open on a clearnet site/regular internet site. But I think the way they get away with it is stating that they are independent without pimps or connections to organized crime. Obviously that is bullshit but the cops generally go after prostitution rings and pimps not independent girls selling their ass out of a hotel.