Auctioneer
I use the advanced suite, because my husband said to, but I'm sure the regular package is all anyone really needs to make a buck on the auction house. I can't set a good price without it, unless I do a search on the auction house for every item in my inventory as I price them. Huge time saver!
Autobar
This bar arranges all your consumables, mounts, pets, and class specific spells (emergency buttons included!) on bar(s)for ease of use. You can use the already made ones, or switch them up to your liking.
Automaton
This makes my repair and bag unloading trips easier. When speaking with the repairman, it will automatically repair if money is available, and sell all grey items. For you that like to collect odd items like a Rogue's Diary, make sure you bank it before it gets sold.
Bartender4
What can I say about this? It makes my clicking area look better. Yes, I am a clicker, unless I am on my hunter or rogue, who have a single button to spam. (cue to point and laugh) It also allows you to put your bag bar and character functions bar and fade them, drag them anywhere on your screen, or just not use them at all.
beql
This not necessarily one I need...but it moves the quest description area to the right, giving more room to view available quests and the quest description.
Cartographer3
Do you like Google Maps? You'll love this.
Gatherer
This add-on shares what you find with others in your guild, and also grabs what they find and puts it on your map. Great for when you need to find a certain herb or mining node!
LynStats
FPS, latency, xp to level, a "new" indicator if you have mail, and time in 24 hour format, all stuck at the top of your screen. YAY!
MrPlow
It shuffles, it stacks, it sorts, it's a plow! It will even organize your bank.
Omen
A threat meter. Have you been pulling off the tank and finding yourself in an iceblock, or worse yet, squishy stuff between the toes of the mob? Maybe you should check this out. This may not be necessary after WOTLK, but now, it's the difference between life and death.
OneBag and OneBank
This skins your bags into one big bag frame.
TomTom
A navigation helper, complete with arrow for us dummies, pointing the right direction to your objective.
TourGuide
This is a great addon for those of us who have already done the first character or two. I don't recommend this for new players, who should be experiencing the lore. This comes preloaded with guides, level per level and zone to zone, to level up easier.
Violation
This is the meter to end all meters. You want healing? You got it! DPS? Here you go. Overheals? Yup. Total damage done? It's all there.
And, last but not least, I give you the best way to get all these add-ons...
WoW Matrix
It helps you download new add-ons and installs them for you, it updates existing add-ons, and has a launch button for when you are done. This, to me, makes add-ons that much less of a chore.
Now, back to your game!