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.