Inuyashiki has a family, a wife and two kids, none of which care about him. When he learns that he has only three months to live, he realizes that the only one who will miss him is his dog. Shortly after this realization, he is killed in a crash landing by aliens. He is rebuilt by them as a machine with a human exterior. How will his life change now that he isn't human?

(Source: Dan of Population GO)

A peasant named Hanbei asks a girl named O-haru for her hand in marriage, but O-haru confesses that she is already thinking of another man named Masakichi from a neighboring village. When Hanbei seeks out Masakichi, he finds that Masakichi possesses a sword and the skills to use it, despite being a peasant.

(Source: ANN)

The first series, 13 volumes -- Ichirō Suzuki, a straight young man who becomes besotted with another guy, Yūki Sato, convinced his bishōnen love is a girl trapped in a boy's body. Interestingly, author Hiroya Oku is one of the characters in the story, the best friend of Satō. The first three volumes of this series collect Oku's early short stories, including prototype versions of both Hen series. (From wiki)

The year is 2028, location in New Koshigaya. A young boy, Isurugi Neru, lives in a poor family and is forced to quit school so that he can find his own way in life. Neru on the other hand, only thinks of one thing: to take revenge on Yagami, a classmate, who's especially gifted and has humiliated Neru numerous times.

Chizuru Yoshida is not your typical high schooler and she's all too aware of it! She has a talent with getting what she wants from her male lovers and often comes across as cold and manipulative...

In truth, despite her experiences with guys, she has never been in love. That's until the fateful arrival of newly-trasferred student, Azumi Yamada, at her school. It was love at first sight for Chizuru--indeed an unexpected turn of events for the story's vixen!

Not only was she in love, but with all things...another girl. Chizuru goes through a brief stage of anger and denial and tries to convince herself that she couldn't possibly be attracted to such a simple-looking, shy, and naive girl! To her despair she finds herself constantly thinking of Azumi, so she finally resolves to become friends with her, but it's not too long when she starts to push things along with Azumi...

Despite the title, Hen is NOT Hentai. But it does have sex, strong language, yuri, and other elements that might make you think "Hentai".

A spin-off manga of the original Gantz series telling an alternate story.

Yamada Yoko, a high school boy who has a movie producer father. One day, while out with friends to shoot a film that he plans to enter into a film festival, a poster of Papiko, an adult video actress with big breasts, catches his eye.

(Source: MU)

Koizumi, Shintarou is 30 years old, a virgin, and has isolated himself in his room for 15 years. He has become a burden on his father, Koizumi Yasujirou. one day, Shintarou makes a proposition that if Yasujirou gets a girlfriend, Shintarou will come out of his room. It turns out that Yasujirou has already made a girlfriend, a young beautiful woman, Yoshinaga Haruka, who is with Yasujirou for her own personal reasons.Yasujirou and Haruka decide to get married, but Yasujirou dies on their honeymoon. Now Haruka decides to take care of Shintarou, and so this weird "mother and son" are now living together.

Thought your life was bad? Sometimes, death is worse. There is no salvation, peace, nor god waiting to receive you into their care. But wait, a god? Maybe you are talking about that big black ball stuck in the room with you. Now you are thrown into a game, fighting green aliens and robot monsters for the chance to survive.

When Kei Kurono is killed, he thus finds himself caught in such a game; a test of his skills, morals and will to survive. His life is not his own, his death spat and trampled upon over and over again. What happens if he does not listen? God knows.

A word of warning: Gantz is not for the faint-hearted, but neither is it as simple as it looks. Gore, rape and violence is rampant, as are portrayals of greed, violence, and all the ugliness that one sees in society today.

(Source: MAL Rewrite)