Knoll makes three great monitor support arms. We'll look at all three of them starting with their best, shown above: The Knoll Rotation. The elbow jointed arm allows for instant vertical adjustment, side to side, and forward and back as the elbow is jointed on both axis. The vertical post also allows the arm mounting height to be adjusted, just not on the fly. Astute observers will recognize this jointed vertical post is very similar to the adjustment mechanism used for Metro Shelves (the adjustable chrome shelves you see appropriated from food service for a number of uses.)
The rest below the fold.
The Knoll Wishbone monitor arm steps back from the Rotation as it does not offer the instant vertical adjustment, but this is their work horse available in several different configurations including multiple monitors, fixed depth, and accessory rail mounting. I love the wire squiggle that manages the monitor cords and keeps them from flopping on your desk.
The last is the Knoll Zorro. The angular profile of the arm is hinged in the middle allowing it to provide forward/back adjustability as well as side to side. When extended fully it makes an attractive diagonal form.