SpartanUI - SpartanUI is designed to free your screen by moving a majority of the interface elements to the bottom. It is the new look for World of Warcraft. Created by Ansu of Dalaran (aka Atriace) and maintained by Beladona. This is an original design developed through painstaking effort and the help of users and fans at spartanui.com
X-Perl - Much enhanced from Nymbia's Perl UnitFrames, and a complete replacement for Blizzard's default unit frames, including raid frames and raid tools, with little remaining of the original Nymbia code.
Bartender4 - a full ActionBar replacement mod. It provides you with all the features needed to fully customization most aspects of your action and related bars.
Auctioneer - 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.
MikScrollingBattleText - MSBT is designed to be an extremely lightweight, efficient, and highly configurable mod that makes it easier to see combat information by scrolling the information on the screen in separate, dynamically creatable scroll areas. It is a replacement for Blizzard's Floating Combat Text and Damage output.
Deadly Boss Mods - DMB lets you know of Boss moves and cooldowns. Very helpful when in a new raid or dungeon and helps you learn the basics. Warnings tell you when to move, stand still, and other things.
Omen Threat Meter - Basically, enemies in WoW decide who to attack by deciding who is the most threatening based on the abilities you use. What Omen does is provide accurate values of your group's relative threat level on individual enemies, so that you can see when you're in danger of pulling aggro (or, if you're next on the snack list if your tank bites it). This info is usually only critical in raids, where only tanks can survive aggro, but it's useful for any multi-player situation.
Atlasloot Enhanced - AtlasLoot Enhanced is an UI mod allowing for loot tables of bosses to be browsed whenever needed within the game. It can access full item tooltips, show drop rates, allow items to be chat linked and to use the dressing room.
QuestHelper - 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 dang Perfect Satyr Horn you can't find. It also computes the fastest path to complete all your quests - and gain experience - as quickly as possible.
Note: Be sure to download all of the other addons that it says if you go with the addon version of the leveling guide. You will need TourGuide, Lightheaded and TomTom. Follow each step and you will be on your way to 80 in no time.