I am a big fan of addons, so far I have tested a lot of them over the years, some better than others, some more essential.
It's always nice to have someone else recommend you an addon, so I will give you my list and why I use the addons in a short description. =)
If you know any good alternatives I'm always willing to try something new.
Advanced Trade Skill Window:
Changes the default trade skill window, it allows you to queu up all materials to craft items and gives you a nice recipe list of needed materials when you go to the AH. (must have)
Puts a grid on your screen so you can align all your UI mods.
Provides you a lot of information about your alts, like time played, gold, inventory and lets you link the professions of your alts while you are on your main (or any other char ).
Athene's Upgrade Estimator:
An alternative to ratingbuster, will also give you points of value in general so you can use lootrank or a spreadsheet to track upgrades yourself.
Database for all in-game loot.
While auctioneer is decent, this addon does more and does it better than auctioneer with far less memory consumption and useless modules. (must have)
Can be set up to track debuffs and/or buffs on a target, I like to use it to track stuns etc.
Replacement for action bars. (must have)
Shows you how much you would block/dodge/parry etc (useful for tanks)
Lists your auctions and shows a total amount of gold worth of auctions.
Tracks cpu usage, memory usage, latency etc.
can track all your money, honor, emblems, cooking awards etc etc.
auto repair, sells grey items etc.
How much time/kills left till next level.
Let's you skin your action bar buttons
Nothing so far I have found has been such an all-inclusive addon to help you quest, even though it's design might be a little clunky nothing gets it done as well as this. (must have)
Tracks my cooldowns. Though I am looking for something more in the style of Elkano Buffbars for this.
Easy dispelling/curing/etc. (must have (if you are a healer))
Deus VoX encounters:
Alternative to DBM, while DBM is a good addon, it drains memory and it feels like DBM has been edited too much on a core that's too old.
Tracks buffs on you, can be set to track for your pet too or an enemy and even your focus.
Simple energy meter with combo points. Nothing fancy, but quite old.
Failbot, whenever someone fails by standing in a fire it will track it.
Alerts when tanks or healers use big cooldowns like pain supression or shield wall. (must have (if you are a healer))
I use it to track if there are any buffs I can steal. Has a timer for your water elemental and some abilities too.
Let's you have multiple item tooltips
Tracks who died and cause of what, also let's you link the spell/ability that was used to kill.
Adds cooldown counter on the button of your ability. (must have)
Tracks achievements, displays in the tooltip of a squirrel that he is already /loved but needs to be killed for the critter achievement, etc.
Most configureable unit frames. (must have)
Opens all your mail for you.
Let's you make all sorts of alerts for all sorts of situations and or abilities etc etc. Can do almost anything.
Best addon for cast bars and for buffs imo. (must have)
Adds the range of your target/focus/pet/mouseover in numbers. (must have)
Tracks what you are saved for and on what character instead of the little annoying default raid lockout box.
Cause default minimap doesn't do it for me.
Tracks if your shadow word: pain has all possible stacks of shadow weaving and crit modifiers (since it refreshes by mind flay you want to have it to to be up with all buffs instead of only a few).
Skada Damage Meter:
Omen and Recount are yesterday, tracks yo threat in combat and your damage after. (must have)
Let's you skin the ugly default skin WoW has to whatever you want.
Though kinda heavy on memory, it sorts all your items for you. I tried almost all bag addons and the auto-sort is just pure win, otherwise I can never find what I got in my bags.
Tidy Plates: Threat Plates:
Skinned health bars that show the health and threat of your target. They resize to your threat, so as a tank when a plate gets very big, it's likely the mob is going for one of your dps. (must have)
Estimates the time till death of your target.
I was thinking TankSpot users might like SmartRes2, which is available on Curse and WowInterface. It makes ressing in groups much faster and easier, with much reduced tripping over each other trying to res the same target. There is a lot more information on the addon pages.