BankItems
An addon that remembers the contents of your bank, bags, mail, equipped, currency, keyring and items up for auction and can display them anywhere in the world. Also takes snapshots of guild banks and characters/banks/gbanks on other servers too. Very handy for checking what mats you have on your bank alt without getting off your main. Also keeps me from having to carry around my off spec gear to compare what I have vs potential upgrades.
It's nice because the snapshots are basically just like the normal pop ups you'd get but without the ability to withdraw. You can still hover over for item stats, etc.
It also adds a line to the hover over for any item showing you how many you have. For instance, if you hover over a Frozen Orb, it will say "2 on Character Firecrest, 6 in guild bank <Bank ALT> - Total 8".
Akis Recipe List
Compare your profession recipes with all known recipes in the game. Useful for completionists and shit like going for that cooking achievement. Also has hover over text for recipes that you don't have, telling you who sells it or what mobs it drops from.
Instant Health
This addon updates the health of player/party/raid members and pets much faster than normal by using the combat log rather then waiting for the server to send updated health info. Normally the health of a unit doesn't update faster than at most 3 times per second and there is often a delay between someone taking damage and the health updating. With this addon the health is updated instantly and the health updates as many times per second as the unit is taking hits and heals.
NOTE: This addon hasn't been updated in some time. I'm honestly not sure if it still works, but when it first came out the difference was very noticeable. I heard in passing once that Blizzard changed their system to do just this with the advent of 3.0, but never found confirmation. The resource usage is minuscule, so I continue to use it.