Dd's first sign was "Mama", of course.
Actually, that sign is thought to have come from the fact that it is one of the first "meaningful" motions that many babies make, similar to the fact that some variation of "ma" or "ba" is one of the first sounds that a baby makes, so most languages have a similar word for "Mama". After that came "Papa", then the first non-parental word was "dog." After that was "milk", "more", "finished", all of that by about 7 months, I think.
I agree about not fully realizing how intelligent little babies are. They can't vocalize, per se, but signing is a great tool for them. My daughter has been making sign sentences for a very long time (I SO have to write this stuff down), and is now telling detailed stories with sign and voice combined. (Just now: "Mama and Papa and [her name] and Grandpa go to beach. Ocean [signs 'ocean' several times, setting scene.] They see Elephant! They play with elephant. They play in ocean. They sleepy. They go nap.")
One thing that's really cool is how she is using grammar, now -- I transcribed the above approximately, but she does some things perfectly in ASL. For example, she will lower her eyebrows while saying "where's Papa's coffee?", then return them to their normal position, nod, and make an "ah" mouth when she finds it, then say "there's papa's coffee!" That's like, "I looked for Papa's coffee and found it right over there!" It's pretty complex. (All "wh-" questions should have lowered eyebrows, an ASL grammatical rule, plus the if/ then structure... argh, I'm not explaining it well, but it was cool to see.)