Awesome Addons

Maps/Zone Info/Transportation

• Cartographer - Cartographer is a modular, lightweight, and efficient framework for manipulation of the world map. Discontinued
• Mapster - Mapster is a very simple world map enhacement addon. It shows unexplored areas and has optional fog of war.
• Cromulent - Cromulent shows the levels for quests, instances, and fishing in each zone. This was part of the original Cartographer.
• Atlas - Instance (dungeon) maps
• FlightMap - Show flight path locations on big map, flight duration during flight

Quests

• QuestHelper - QuestHelper tells you how to finish your quests in the easiest, fastest manner. This is essential for new players looking to actually level.
• LightHeaded - Displays quest information and comments from WowHead in game
• Bunny's QuestLevel - Adds level to quest name in the quest log (Replace with LevelDisplay)
• LevelDisplay (Levelland) - Adds level to quest name in quest log, gossip window, and links

General

• GatherMate - Collects Herbs, Mines, Gas Clouds and Fishing locations and adds them to the worldmap and minimap.
  • GatherMate_Data - WoWDB Data dump for GatherMate
• Auctioneer - Auctioneer Suite provides you with the tools and data necessary to make those difficult auctioning decisions with ease.
• AuctionLite - I haven't used this, but it seems like a lightweight Auctioneer replacement
• OmniCC - Adds numeric display of cooldown timers

Raid/Party/Social

• Deadly Boss Mods - If you go into a dungeon or battleground, you need this. Period.
• Deadly Boss Mods - Vanilla and BC mods - DBM for Vanilla and BC dungeons. (Some people still do older content!)
• Recount - Damage/Heals/etc. meter; Ranks everyone and breaks down what spells/attacks caused what percentage of damage/healing
• Classic ThreatMeters - Allows you to see the threat bars Blizzard had originally implemented in WoW, but disabled and removed with a patch down the road.
• Omen Threat Meter - "Omen is a threat meter." Some people prefer this instead of Classic ThreatMeters. They both do about the same thing.
• Grid - Grid allows for an extremely customizable view of everyone in the party/raid. I find it immensely helpful for healing and tanking.
  • GridAutoFrameSize - Sizes Grid based on number of actual people -or- based on proper zone size
  • GridIndicatorSideIcons - Adds 1 icon to each of the four sides (top, bottom, left, right)
  • GridStatusDungeonRole - Adds indicator to show roles (tank, heal, DPS) when the dungeon finder is used
  • GridStatusMTs - Adds icon or "MT" text indicator to Grid to mark main tanks
• Grid2 - A rewrite? of Grid. The author has stated that it should be considered Alpha quality.
• Clique - Clique is a simple GUI that lets you assign click-casting for any number of unit frames. This is a great companion for Grid!
• VuhDo - Raid frames somewhat similar to Grid. VuhDo has click-casting support built in.
• FriendsWithBenefits - Synchronize your friends lists across all characters
• Badapples - Badapples maintains a list of players who you have deemed to be "bad apples." IMPORTANT: As of version 3.2.0, you will need to apply the fix mentioned by Cosmic Cleric (see the post dated 12-08-2009).
• GearScoreLite - Unfortunately, many players now place a high importance on "Gear Score," so you'll likely want to know the gear scores of yourself and others. This is a less bloated version of the full addon. IMPORTANT: As of build 303, you will need to apply the fix mentioned here.

UI Mods

• HideBlizzard - Hides parts of the default interface. If you just want to remove the gryphons, use the Gryphons Remover addon below instead.
• Gryphons Remover - Removes the gryphons from the action bar. This addon is less than 1KB!
• kgPanels - Lightweight plugin to create panels to customize UI
• SLDataText - Lightweight text-based display of numerous pieces of information
• X-Perl UnitFrames - Major UI replacement
• IceHUD - Player and target health/mana/casting bars in the middle of your screen
• ClassPortraits - This addon shows a picture of the class instead of a portrait. This works on yourself, your party, and players you target. This is very helpful for PvP!
• Extra Bars - This will allow you to add a new Action Bar of a chosen size anywhere on the screen (even vertical orientation).
• Tipsy - Tipsy is a simple tooltip repositioning addon. Nothing more, nothing less.
• FizzleInventory - Adds color borders around items indicating their quality (green=uncommon, blue=rare, etc.)

PVP

• Gladius - Gladius adds enemy unit frames to arenas for easier targeting and focusing. (Not updated for 4.0 yet)

Achievements/Completion

• SilverDragon - Notifies you of rare mobs
• Collect Me - Collect Me keeps track of your companions (minipets) and mounts in the game and can show you your missing pets and mounts with additional information where to get them. It also can summon companions randomly with a priority list.
• EveryQuest - EveryQuest is a Quest History tracker and quest list window.
• Overachiever - Tools and tweaks to make the lives of players seeking Achievements a little bit easier.
• Ackis Recipe List - Scan your trade skills and provides information on how to obtain missing recipes.

Class Specific

• Paladin (Holy)
  • CLCBPT - Tracks Beacon of Light, Sacred Shield, Flash of Light HoT, and Judgements of the Pure with (customizable, positional) timer bars similar to DBM. The bars show who is affected, and a configurable warning lets you know before they expire.
  • HealMinder - Tracks Flash of Light HoT and alerts you when it is time to reapply it. I like this better than the CLCBPT bar because it shows you how much healing is left, rather than the duration. (Sacred Shield removed in patch 4.0.1)
• Shaman (Enhancement)
  • ShockAndAwe - ShockAndAwe displays timer bars for Shocks, Stormstrike, Windfury (internal CD), Maelstrom Weapon, Lava Lash, Feral Spirits, and your shield. This is the de facto best addon for an enhancement Shaman.