I think teaching my son sign language is the single, best parenting thing we have done. I really look forward to seeing how our daughter-to-be handles it.
We couldn't imagine the crying, frustration and misery that we would be going through if he couldn't even do rudimentary communication.
We got hooked on this from a friend and she exposed us to Signing Time. There are probably many other good videos and such out there, this is what we hit on first. I think I have the entire series tivo'd from our pbs station.
About a month ago my wife sat down and tried to determine how many signs he knows. It was over 60. He was about 16 months old at the time. He spoke maybe 6-8. It is amazing the capacity for communication our species has.
I can not recommend strongly enough that if you have a young child that you should teach it sign language. I think it defuses frustration and lets you know what they want, at least earlier and more precisely. Otherwise they are left to pointing, grunting and crying until they have the verbal skills. It is just amazing to see his language develop and change daily if not hourly.
It was the most amazing thing this weekend when he signed "please". Well, I guess the list is long about the amazing things small humans do as the grow, especially when it's your genes doing it.
Edit: Talking with my wife, we started signing to him around 6 months. Mostly simple things like: more, eat and milk. He didn't start signing back until he was 10 months old, which made us wonder if the whole signing thing was working. Now at 17 months (almost) he is signing a new word every day.