The main World of Warcraft Addons that are used are usually;

Deadly boss Mods or DBM
This addon allows you to understand the tactics of a boss/raid encounter. Whether you are making a mistake, e.g. You are standing in Fire. DBM Will send you a note in the Middle of your screen telling you to move, or you'll basically die. This addon is also used for PvP, It can help you calculate what you need to do to win, If you are going to win or lose in a BG, and to help you with your damage control, and the damage you take.

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EPGP (Loot system)
EPGP Lootmaster provides a nice GUI for distributing loot and GP to the raid.

It sends a popup and a message to all raid members with the question wether they want the loot or not and sends this response to the master looter. The ML gets a nice list of every raider's choices sorted by EP/GP ( Effort Points/Gear Points ).The person with the higher PR gets the first say on loot. As a result, the more effort you put the more gear you receive. As a matter of fact, the gear you receive is proportional to the effort you put. EPGP is based on this simple principle.

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Quest Helper
Questhelper tells you how to finish your quests in the easiest, fastest manner. It includes a database of quests, monsters, and items, telling you the exact location of that item you cannot find. It also computes the fastest path to complete all your quests - and gain experience - as quickly as possible.

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Recount
Recount is a graphical damage meter which many players use to improve their ingame play. It shows you how much Damage you are doing, and your Damage Per Second (DPS) There are also many other options to choose from once the addon has been installed, such as Heals per Second (HPS)

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Interface (New UI)
The newest and most commonly used interface is the LUI. I am currently using this myself, Here is a print screen image of my Interface using LUI, The colour (being green on this character) changes depending on what class you play.

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Healbot
HealBot allows for 60 key mouse combinations to cast any beneficial spells.
HealBot allows for 60 key mouse combinations to cast on disabled bars.
Any usable item can be added to all combos.
Any existing macro can be added to all combos.
Target, Focus and Assist can be added to all combos
HealBot can track and display who has aggro
HealBot can be used for decursive functions
HealBot can be used for monitoring buffs and buffing
HealBots appearance can be configured and preferred appearances can be saved as skins.
HealBot can show party/tanks/checked targets/extras in a raid, extras can be filtered and sorted.
HealBots in party/raid will sync incoming heals with other healers.
HealBot has a built in res monitor and is compatible with CTRA (CTRA not required)
HealBot has a SmartCast out of combat option, this will cast a spell in the following order..
1.res
2.decurse
3.buff
4.heal
5.user setting

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Gatherer
Gatherer is an addon for herbalists, miners and treasure hunters in World of Warcraft. It's main purpose is to track the closest plants, deposits and treasure locations on your minimap.

The addon does not track like a tracking ability does, rather it "remembers" where you have found various items in the past. It does this whenever you gather (perform herbalism, mining or opening) on an item, and records the specific map location in its history. From then on, whenever the item comes into range of being one of the closest 1-25 (configurable) items to your present location, it will pop up on you minimap.

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Auctioneer Suite
Auctioneer Suite provides you with the tools and data necessary to make those difficult auctioning decisions with ease.

From purchasing to posting, the Auctioneer Suite provides time-tested tools to allow you more time to actually play your characters or play the market. As you will soon discover, it is much more than a simple "Auctioning AddOn", the Auctioneer Suite also shows in-depth information for all game items, such as those used in quests or recipes, enchanting, milling, prospecting and more.

Numerous other AddOns will also use the Auctioneer statistics you gather via scanning. This is especially true for many of the popular crafting AddOns, such as Little Sparky's Workshop.

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Addon Control Panel (Must Have!)
ACP adds the "Addons" button to the game's main menu (The one you get when you hit ESC). It allows you to manage your addons in game, with an interface which looks similar to the blizzard addon manager. ACP will help you deal with the "Clutter" that multi-part addons and libraries introduce by displaying your addons in logical arrangements. ACP has many features to make your addon list easy to manage, help you with missing libraries, and provide you with detailed information about each addon.

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How to add addons to your World of Warcraft game.

1. Go to here on your computer C:\World of Warcraft
2. Go to Interface, Addons.
3. Add the addons that you want into that folder.

I shall be updating this post when I find and use some more useful addons and think that others may benefit from them.