ControlFreak's Guide to WoW Must-Have AddOns

[ControlFreak]

Auctioneer: Helps to know the going rate of items and how to price them in the aution house
Bagnon: Combines the inventory bags into one bag, and does the same for the bank
Bartender4: Gives options and additional functionality for all hotbars (allows up to 10 hotbars)
Crap Away!: Automatically sells grey items (crap) when you open a vendor window
Deadly Boss Mods: Provides alerts for just about every boss in the game
DropTheCheapestThing: Allows you to drop the least valuable item if your bags are full
EventAlert: Pops up an alert when an event happens, usually based on Talents.
OmenThreatMeter: Allows you to see a graphical display of "threat" for the group (helps you to know if you're about to pull aggro from the tank)
Postal: Gives additional functionality for the mailbox
Quartz: Gives some casting bar enhancements, including a good view of what your target is doing
Recount: DPS meters (loads of other meters too)
TitanPanel: Gives a bar (or 2) across the top of the screen (or bottom) that can contain a huge amount of useful (all configurable) data. There are many extensions for this
VuhDo: Gives a grid of "cells" for the people in your group/raid. Allows you to map mouse clicks for this grid and can show other useful information
X-Perl Unitframes: Allows you to fully customize the unitframes in the game (player, target, pets, group, raid, etc)

Those are the "must-haves" ... here are the "might-wants" grouped by category

Crafters
Advanced Trade Skill Window: Title says it all
Gatherer: Tells you where the stuff is for mining/herbalism
Recipe Radar: Let's you know if there are recipes for your crafts available in the current zone (or which zones to go to)

UI Enhancements
ChatSounds: Let's you set sounds to play when certain types of chat are used
Parrot: Enchanced "Floating Combat Text"
Procodile: Let's you know when something "procs" (ie, armor/weapon)
SexyMap: Adds a bunch of options for your mini-map, including some graphical styles

People like me
_NPCScan: Let's you know if there's a rare spawn nearby
_NPCScan.Overlay: Tells you whee rare spawns are in an overlay on your map
Altoholic: Let's you track a huge amount of information about your alts

[ControlFreak]

Here are the ones that didn't make the above lists:

Atlas: Gives maps of everything .. instances, zones, flight paths
Atlasloot Enhanced: Tells you what items bosses can drop
Broker_Portals: Gives a button that has all ports for mages
GathererDB WoWHead: A full database for Gatherer with about 20,000 locations of mines, herbs, and other crap
GearScore: Gives a score based on your current gear. This is used at level 80 to know what raids you should be able to do
QuestHelper: Gives an arrow that points at the next item for quests. Not quite as "must-have" as it used to be now that default WoW gives locations on the map
RatingBuster: Calculates the effects of items vs your worn items to give a better apples-to-apples comparison
SimpleMarker: Gives an easy way to mark targets (skull, X, diamond, etc)
Titan Panel (Guild): Gives Guild info for Titan Panel
Titan Panel (Pals): Gives friend info for Titan Panel (who is online, etc)
Titan Reputation: Shows your reputation (and % to next rank) based on the last reputation hit you got
ViperNotify: Tells hunters if they need to turn on (or off) Aspect of the Viper
WIM: Kinda like AIM for WoW. Gives windows for all chat comversations .. must haves for people who talk a lot