There's the self-generating harem, where the main character does one girl, she tells her friends, and the next day there's twenty of them at his doorstep.
There's the mega-harem, where one guy finds himself stuck with hundreds of girls who refuse to serve anyone but him.
One I call an Ultimate Harem, because it is absolutely unlimited. The main character isn't cool or anything, but he has something about him that, whether he wants it or not, makes every girl he encounters go absolutely insane with love or just simple lust. DNA² is a great example here.
There's also the Harem Academy. This happens a lot, both in hentai and normal. A prestigious, high-standard, all-girls school located some distance from the cities becomes co-ed. Thanks to location, standards, and how quickly it changed, few boys know about it, fewer want to apply, and far fewer are accepted. The result is a girl/boy ratio of about 1000/1.
Ultimate Harem Academy combines the last two. A guy transfers to a school that is otherwise all-girl, and he has a secret. One example I found was Daisuki Daisuke, a series of shorts from Yuuki's Kare to Kanojo no Jijou (note it's not "kareshi).
Spoiler (highlight to view)
Daisuke used to go to an all boys school, but he left, probably because the guys wanted him dead. The girls are beside themselves with anticipation, but when he comes in their expressions turn to those of a person who bought a Jaguar and got a station wagon. Daisuke needs to be careful, because when he's "excited", he produces a pheromone that instantly brings women to the peak of arousal, while also creating minor hallucinations that complement the feeling. The first girl to experience this slams him to the wall and asks what he did to her, then says it doesn't matter and proceeds to go for his pants.
Harems are very entertaining, and very funny, and stuff like Daisuki Daisuke can be downright hilarious, since it forces the man past the boundary between a heaven of eternal pleasure and a hell where he never rests and longs for the day he dies of exhaustion. Ultimate Harem and Harem Academy are both welcome, but especially looking for combinations. If anyone has anything, especially translated, please post.
