Here are some highly recommended addons that will make your game a lot easier. Please ensure that you have the ones marked as required addons or you will not be able to raid.

* = Updated for 3.0.2

Atlas Loot *(Dungeon loot info for bosses)

Auctioneer Full Suite *(Track AH prices and get deals/steals. This version includes Auctioneer, Auctioneer Advanced, Enchantrix and other related mods.)

BaudBag *(One big bag instead of many small ones)

BaudMount (Automatically equip your carrot and boots with spurs, or perhaps your riding crop etc, when you mount, and then put your original gear back on as soon as you dismount. EXCELLENT mod, highly recommended.)

Cartographer *(Map mod. Create and share map notes.)

Deadly Boss Mods *(Bars/Timers for boss abilities - REQUIRED for endgame raids.)

Decursive *(Remove magic, curses, disease, etc. easily. Any class that can remove any debuff should have this. REQUIRED for priest, pally, mage, druid, shaman. Locks can also use this for their Fel Hunter's devour magic ability to remove a negative magic effect from yourself!)

FastQuest Classic (Automatically notify party members of your current quest status in the party chat window. No more "How many more Naga claws do you still need, dude?")

Group Calendar *(In-game utility addon for raid scheduling. Also provides notices to you when your cooldowns for tradeskills are available, even on different toons. REQUIRED for guild raid scheduling.)

GUILD2GUILD *(Allows main guild and alt guild gchats to seamlessly link together. Once you install it, you'll see guild chat from both as if they were both "in the same guild". REQUIRED for all guildies, even if you don't have any alts.)

GrudgeBook (You know that horde that just ganked you? How about that idiot in your PUG that wiped the party 10 times? Record your grudge <ganker, beggar, jackass, noob, etc> and your grudge will show up whenever you target them. If you're feeling particularly vengeful, it will also tell you the last known location of the person from data gathered from others with this addon.)

Lightheaded *(Like a built-in thottbot for your quest log. Not required, but you're a fool if you don't use it to cut your questing time in half.)

Mob Map (Database of all items, NPCs, and quests in the game. Requires the database below. Handy addon if you have enough memory for extra goodies.)

Mob Map Database (Required data for Mob Map above.)

Omen *(Threat meter - REQUIRED for endgame. Highly recommended for everyone else. Know exactly how much DPS you can do without pulling whoever the tank is holding)

Ventrilo *(Voice chat program. REQUIRED for all raiders. This is a separate program [not an addon] and does not need updated for WoW patches etc. You want the Client version, not the server version. Oh, and for the Windows users... you need the 32-bit version of Windows or you are such a geek that you would already know this stuff and wouldn't be reading this.)

X-Bar *(Small moveable bars. One for tradeskills, one for each class, etc. Very handy addon, highly recommended.)