ErgoGlide is an interesting keyboard tray (sort of) I saw advertised in an ergonomics magazine. Created by an Arizona chiropractor, ErgoGlide supports the keyboard and moves up and down and in and out. "ErgoGlide™ is a mobile hand, wrist and forearm support system that allows keyboard users freedom of movement across the keyboard in both the “x” and “y” axis." I imagine this allows you to get just the right height so your wrists aren't tilted up or down when you type, but can rest in the recommended neutral position.
Videos and more information are available at http://ergoglide.com/