Most people type in relative silence, but there's always a few noisemakers - those people who type not by pressing keys but by smashing their fingers down on them like little ball-peen hammers. Buy them Thanko's Keyboard Quiet Cover.
The cover comes from the ever-ludicrous Thanko and reduces noise from keyboard clatter. A nobile aim! A clear plastic window lets the typist see what they're doing when they're going for some of the tougher keyboard maneuvers, like stretching the pinky out in search of the tilda. That's always a tough one.
The cover also offers the added benefit of protecting your keyboard from water and dust, though it has the added drawback of making one an inevitable target of office ridicule. But such is life. You can get one for the loud typist in your life for ¥2980, about $36. [GeekStuff4U]