For example, one of the addons I simply could not do without is Baud Manifest. This little guy completely eliminates the standard "inventory-as-icons" motif and just gives you a list, with searching/filtering/custom categorizing features available. If you're like me and remember what something's called long before you remember the icon, then definitely give this one a try. It also works seamlessly in the Mists beta, which makes me a happy panda.
What do you think? What are your favorite addons that nobody else knows about?