Josh and I got a book as a gift about teaching babies sign language that we are really excited to read. It talks about why you should teach your baby to sign and what signs to teach your baby.
My brother and sister in law did this with their son Cole and it was awesome because they could communicate with each other before he was able to talk. It reduced the frustration of him knowing he wants something, but not being able to ask for it yet. For instance they taught him "please" first so if he wanted something he pointed and signed "please" then they started teaching him signs for things he would typically ask for like "milk" or "juice". They taught him "done" and "hungry" and "sleep". It was great for all of us to learn how to use these signs to communicate with him.
Teaching a baby sign language can start around 6 months. Josh and I really want to start earlier with Mia so she is used to seeing it so that when she's ready to use it hopefully it will be easier for her to pick up on. Babies won't sign back for a while certainly not before 6 months, but usually around 8 months their motor skills are developed enough for them to make their first sign.
A lot of people wonder if teaching a baby sign language will delay their speech, but that is not likely. I know in the case of my nephew he stopped using sign once he could say the words and wasn't delayed at all. He actually linked the words with the gesture because mom and dad always said the word out loud when he signed it so by the time he could say it I guess he figured he didn't need the sign anymore.
Did you use sign language with your baby? How did it go?