It's my impression (as someone who doesn't know sign language) that not only are there several, as Karyn points out, but that deaf children learn sign language from signing parents in the same way that hearing children learn spoken language.

As Steven Pinker says in The Language Instinct, deaf children who learn sign language only as adults "must struggle with sign language...much as a hearing adult does in a foreign language class."

I think that early language instruction (e.g., in elementary school, still very rare here in the U.S.), combined with frequent use of whatever language that is, offers great promise.