Martha's Vineyard Sign Language was a signed language used for almost two hundred years in the small island off Cape Cod. In Martha's Vineyard in 1854, 1 in 155 residents were deaf. Both hearing and deaf people in the community used sign language, to the point where a person's hearing ability was unimportant because everyone could sign. They discovered the ability of a signed language to be of assistance in everyday situations like trying to communicate over long distances or when silence was necessary.
In 1817, the American School for the Deaf opened in Connecticut. Children from Martha's Vineyard quickly enrolled at the school, bringing with them their sign language. This, combined with the teachers' use of French Sign Language, and other children's home-crafted sign languages, combined to create what is now American Sign Language.
Martha's Vineyard Sign Language died off as Martha's Vineyard became a tourist-oriented island. However, it remains a fascinating point in the development of language.
You can read the book Everyone Here Spoke Sign Language: Hereditary Deafness on Martha's Vineyard by Nora Ellen Groce here on Google Books , discovered thanks to Boing Boing's article on this topic . I am reading my way through it right now; it is fascinating reading about the adaptability of humans to their environment.