No eroge novels (aka 'erotic visual novels') in this list: there are so many of them that we have made a page just for them.
We also have a page for Adult interactive Fiction for smut made by non-Japanese writers.
Original name is かりんとあそぼ！, also remember to set your PC on Japanese Uni-Code. Very good gameplay, with collectible skills and weapons and some very hard jumps.
The game is in English, but there's no text apart from the start screen and the in-game menu.
A very simple game at its core, Kare Kano's gameplay consists of a very limited traditional VN dating sim style, where the protagonist chooses actions for the day that affect his statistics.
The actual sex simulation gameplay is pretty standard. You can browse the online repository of other players' heroines and date, then ultimately steal them. The game's dialogue is very mediocre and unimportant. The game also comes with one of the best character creators on the market, also featuring online options for finding more outfits and features.
Genre feels like a fusion between an original simulator and an adventure game.
Contains dungeons filled with enemies, secret rooms, weapon upgrades, and bosses.
ZONE games aren't really games, but sweet Jesus does 4chan love them and each one is well-made. Another Illusion game, so it's good stuff as usual.
You can create a classroom with 25 students complete customizable (the male customizations are really poor compared to the female ones) and then see how they react with each other.
The demo goes until the first boss and gives a good idea of the game.
You can get it and the full game from the author's blog: You play as Raidy, an adventurer, wandering the continent of Else.