Here's a link to the refurbished lenses section of the Nikon store. Some good deals, some not-so-good deals. I just bought a 14-24mmf2.8 lens for a good deal.
The Canon refurb store has good items for Canon shooters:
I am buying a lot of refurbished stuff these days, from woot.com to the Canon refurb store, to other places. Never a problem except perhaps for my Western Digital media players. I would recommend against buying refurbished hard drives.