Ranma 1/2 is about a boy who, while in China, falls into a magical spring which causes him to become a girl everytime he is splashed with cold water. His friends and acquaintences also suffer from various cursed springs. The series is 30-some books long.
Images are courtesy of Marci.
Manga Manga Manga!
Ranma 1/2 is © Takahashi Rumiko