[Ichigo3924]

Well, I can add to this, still can't believe that THIS classic wasn't added first though!

Ranma 1/2: Really, everyone knows about Ranma 1/2 already, but for those who don't I suppose a summary would help. So, on a training journey in the Bayankala Mountain Range in the Qinghai Province of China, Ranma Saotome and his father Genma fell into the cursed springs at Jusenkyo. When someone falls into a cursed spring, they take the physical form of whatever drowned there hundreds or thousands of years ago whenever they come into contact with cold water. The curse will revert when exposed to hot water until their next cold water exposure. Genma fell into the Spring of the Drowned Panda while Ranma fell into the Spring of the Drowned Girl.

Upon returning to Japan, the pair settle in Nerima, Tokyo at the dojo of Genma's old friend Soun Tendo, a fellow practitioner of Musabetsu Kakutō Ryū or "Anything-Goes" school of martial arts which Genma passed on to Ranma. Genma and Soun agreed years ago that their children would marry and carry on the Tendo Dojo. Soun has three teenaged daughters: Kasumi, Nabiki and the hot-tempered, but helpful, martial arts practicing Akane. As Akane is Ranma's age she is appointed for bridal duty by her sisters. Their reasoning is that Akane dislikes men (she was aggressively pursued by most of the boys in school at one time), and that Ranma is only a man half of the time; therefore, they are perfect together. Although both initially refuse the engagement having not been consulted on the decision, they are generally treated as betrothed and end up helping or saving each other on numerous occasions. They are frequently found in each other's company and are constantly arguing in their trademark awkward love-hate manner that is a franchise focus.

And yes, I did just copy and paste this from Wikipedia, go ahead and hate me.

Sekainohate de Aimashou: The general plot of the story is that a boy (yes, “he” is a boy) saved a man from outer space and the man turned the boy into a girl by using some kind of alien technology/potion and force “her” to be his wife. Who’s the boy? He's a normal high school student who looks like a girl and is liked by his brother. Who’s the man? He's the prince of one of the most powerful empires in the universe.