The board books we used taught a simplified version of American Sign Language (some signs were actual signs others were watered down a bit). They had bright photos of the object and one of a child doing the sign, with simply written instructions for the parent. It was very easy to teach. I also bought one of the larger signing with your baby kits - we never used it. For basic communication, the board books ruled and were a lot cheaper.
Sophie stopped signing quickly after she achieved the verbal skills to speak. It was bittersweet to see it go. Her friend, Carolyn, was taught American Sign Language and her parents spent a lot of time giving her an extensive signing vocabulary. At two, she still uses her signs, along with vocal language. I have a feeling, Sign Language will actually be a second language for her. Which would be great. I'd imagine that alot of the benefits of teaching a second vocal language also applies to Sign Language at this level. Of course, I am just guessing about long term benefits outside of self esteem. There is a new article out that talks about the benefits of signing with your child.
Sign language class helps babies communicate
...Learning simple sign language not only gives babies a way to communicate other than whining or crying, research also indicates it enhances language development, gives a good foundation for early literacy and can stimulate intellectual development
I am excited this is catching on. We will definitely teach it to our newborn.