My Grandfather had one and used to let us check our homework with it. It didn't do more than add or subtract, but it was a neat toy.
Added: The one I want is the Curta (http://www.darkroastedblend.com/2008/09/stunningly-intricate-curta-mechanical.html). I've seen them used, but never used one myself.
Keep Up with TechRepublic