King Duncan, Lady Macduff, and Banquo. The suggestion is that Lady Macbeth is too guilt-ridden to maintain silence, though when awake, she forces herself to do just that. Shakespeare well knew that conflicts have a way of working themselves out, or at least atempting to resolve themselves, when one is asleep.
Freud said that there are no accidents; perhaps Lady Macbeth wanted to be caught. We never find out, however, She is dead soon after this scene.
One of the more fanous lines from Shakespeare occurs here: Out, out.... This is echoed by Macbeth later, and of course, used as the title of a Frost poem.