We just received this great-looking book from some forward-thinking friends. Turns out sign language for hearing babies is quite a cottage industry. [Kindersigns is just one of many programs online.]
The basic concept: babies as young as 6-7 months can learn to communicate via simple signs, months before they're actually able to speak. This is supposed to reduce fussiness and stress. Of course, by 6 months, a baby crying should be like crickets in the summer; pleasant background noise.
We used sign language with baby #1. The 3 signs "milk", "eat" and "more" helped a lot in figuring out what he wanted. Plus, it's a kick the first time you see your child communicate with you -- my wife and I looked at each other and said, "Did he just sign 'milk'?!!" (Son was about 5 months old at the time.)
I recommend the Signing Times videos. Even though the production values are a bit low, they are filled with little kids which keeps the interest level high for infants and toddlers.
Our nanny's daughter, 11 months older, was not much of a talker at the time. She enjoyed the videos and picked up the signs quickly.
have a look at the website www.weehands.com for some good information and links on the topic of signing with your baby.