This looks like a Golden Lion Monkey - they are pretty interesting. A couple of months ago, the Kansas City Zoo had an 8 week old baby that would hop on an adult's back for a ride around the exhibit. I think the National Zoo in Washington, DC has it arranged so that the monkeys can travel over a large part of the zoo, but they remain inside wiring so they can't escape.
He is adorable. And he does look as if he's going to throw something. Or maybe he's posing, No! I think he's looking at something. Not the photographer. Look again!
:)