Canon Lens Hood EW-73B for EF-S 17-85mm f4-5.6 IS USM
There's no reason not to purchase this. If you invest in a lens, invest in a solid hood for it. Not only does it protect your lens glass from accidental drops, but it does a great job of keeping unwanted light from entering the camera.
This hood is sturdy, has a nice velvet coating on the interior and is easy to remove/attach. Stores nicely in the backwards position. Forget those cheap $5 knockoffs.