Amanna has mentioned an awesome little tool for tanking Druids with a great name: Rawr. It's a completely separate application for Windows (sorry Mac users) that will pull down your gear from the Armory, and help you not only see your tanking strengths and weaknesses, but help you choose upgrades, both in terms of gear and gems on it.
Astrylian of Kilrogg put it all together, and it looks like a really amazing tool for bears. Unfortunately, no other classes or Druid specs are supported yet (Astrylian says cats are next on the list), but if you're a bear who's been trying to make some good gear choices lately, this could be just what you need. Rawr is still in beta and available as a free download on the Druid wiki.