Bartender4 - a full ActionBar replacement mod. It provides you with all the features needed to fully customize most aspects of your action and related bars. Includes an out of range indicator for buttons.

Buffet - generates macros that will put the best food/water available into a button, the buttons will automatically switch to health and mana potions when in combat. You can press shift to see bandages.

Clean Icons - Mechagnome Edition - replaces the default Icons with a better look.

LiteButtonAuras - Shows your buffs on you and your debuffs on your target inside your action buttons with a colored border and timer. You can see an example on the Blessing of Might in the image.

OmniCC - adds cooldown text to any button with a cooldown. You can see the cooldown on Lay on Hands in the image.

Addon Manargl - Addon manager that has Filters, Grouping by dependencies, Profiles, Autoload profile, ElvUI Skin, And many other flexible settings.

Auctionator - Designed for casual everyday auction house users. Auctionator makes the auction house easier to use by presenting auction house listings clearly and succinctly and by eliminating the tedium involved in posting and managing auctions.

TradeSkillMaster - I use this addon to post my auctions and keep track of item prices over time. In the screenshot you can see I am keeping track of historical prices along with market value and current minimum buyout. In order to get the prices in the tooltip you will need to install the TSM Desktop Application. Once installed it will automatically detect your classic era server.

Baganator - A comprehensive and straightforward single or categories view bag and inventory addon. I use this addon in categories view. You can create custom categories.

WeakAuras - Allows the display of customizable graphics to indicate buffs, debuffs, and other relevant information.
This addon has a learning curve so it may not be good for a casual player, however there are many premade weakauras that you can import at wago.io. If you want to make your own weakauras check out the WoWHead Guide.
Here are the WeakAuras I use:
Avoid Lava Damage - Shows when to jump to avoid lava damage in BRD.
Circle Cursor - Puts a circle around your cursor making it easier to see. I have mine set to only show in combat.
Luxthos Experience Bar - XP bar with options.

Buffwatch Classic - Buff monitoring tool for all classes which lets you easily and clearly keep track of any buffs on yourself or your party/raid, and also allows you to quickly recast your buffs at the click of a button.

vChat(TBC) | vChat(Era) - This started as an updated rChat by Zork but by the time I got done alot had changed. Features: custom font with outline and shadow, editbox to top, mousewheel scroll, short channel names, class colored player names with no brackets, keybinds for guild/party/raid and say, hidden chat buttons. You can change the font at the top of the Lua file with a text editor.

Cleaner Loot Messages - Reduce loot messages to a minimum for clearer information.

devnull - When you enter a capital city it will ignore /yell, spammy NPCs, duel spam, emotes except those directed at yourself, drunk spam and tradeskill spam.

Incognito Resurrected - Adds your main name in front of your chat messages when on alts. Can be enabled for guild (and officer), party and raid chat messages.
[Guild] [AltName]: Some chat message
becomes
[Guild] [AltName]: (MainName): Some chat message.

Bazooka - Adds information bars on the top or bottom of the screen which provide data broker plugin support.

Broker Everything - A collection of modules to display various information in game about the game. Modules I use: Gold, XP, Professions, Durability, Volume, Clock.

I use Proxima Nova as my main UI font and Proxima Nova Condensed for chat. These are paid Adobe fonts but if you search online you can find some free downloads. For directions on replacing game fonts go to the Fonts page.

DeModal - Makes the Blizzard windows movable.

Extended Character Stats - Lists all the available stats not displayed in the character info pane by default. You can config per character all the values you want to see and hide the ones you don't care about.

Outfitter - Equipment management addon which gives you fast access to multiple outfits to optimize your abilities in PvE and PvP, automated equip and unequip for convenience doing a variety of activities, or to enhance role-playing. If you just want to automatically equip faster mount gear try AutoCarrot.

Simple Item Levels - Show item levels on: The character panel, The inspect panel, Weapons, armor, and artifact relics in bags. Shows missing enchants and gems on character panel.

Clean Icons - Mechagnome Edition - Replaces the default game icons to change the look without using a mod.

SharedMedia - Inserts media(textures, bars, borders, etc) into LibSharedMedia's storage. They can then be selected by any addon that supports LibSharedMedia, a lot of them do. I have collected many fonts and statusbar textures which I insert into my own SharedMedia for my UI.

Clique - Simple addon that enables click-casting and hover-casting on your unit frames and in the game world. You can bind virtually any mouse or keyboard combination to a spell or macro. In it's normal configuration this enables you to use the bindings over your unit frames in order to cast spells directly on that unit. This allows you to more quickly select both the spell to cast, and the target of the spell without requiring an extra click.

Cell - Cell is a raid frame addon inspired by several great addons, such as CompactRaid, Grid2, Aptechka and VuhDo. With a more human-friendly interface, Cell can provide a better user experience.

Leatrix Plus - A collection of game settings. There are many available settings, I use the following: Automate Quests, Automate Gossip, Sell Junk Automatically, Repair Automatically, Block Duels, Hide Combat Log, Block Drunken and Duel Spam, Hide Error Messages, Enhance Quest Log - Double Wide, Enhance Trainer Frame, Show Wowhead Links for Quests, Show Flight Timer Bar, Manage Buff Frame, Set Weather Density, Max Camera Zoom, Mute Various Game Sounds, Faster Auto Loot, Faster Movie Skip, Easy Item Destroy.

Syndicator - Shows in tooltips where your items are in all your characters bags, banks, mail, auctions, and all your gold and currencies. In the screenshot I have a total of 460 Mistletoe spread across 5 characters bags and banks.

Clam Pulp - Automatically opens clams once looted.

LootRollMover - Allows you to move the default Blizzard Loot Roll Frames.

Ground Clutter Macro

Clicking it will alternate between no ground clutter and high ground clutter. This is great for finding herbs or quest items that can be hard to see in tall grass.

/run local g="graphicsGroundClutter";if GetCVar(g)=="0" then SetCVar(g,9) else SetCVar(g,0) end;

Left/Right Click on One Button

Left click will cast one ability, right click will cast the other. In the example left click casts Blessing of Might, right click is Blessing of Wisdom.

/cast [btn:2] Blessing of Wisdom; Blessing of Might

Modifiers on One Button

Click will cast one ability, holding shift or control while clicking will cast the others. In the example click casts Teleport: Stormwind, Shift-Click Teleport: Ironforge and Control-Click Teleport: Darnassus.

/cast [nomod] Teleport: Stormwind; [mod:shift] Teleport: Ironforge; [mod:ctrl] Teleport: Darnassus

Weapon Swap Macro

For Paladins and Warriors, this will swap your 2H weapon with your 1H/Shield. Works in combat.

#showtooltip
/equip Name of Shield
/equip Name of 1H Weapon
/equip Name of 2H Weapon

Gnomish Inbox Shrinker - Replaces the default inbox UI with a minimalistic, scrolling list that gives you a much better overview and no paging.

Postal - Offers enhanced mailbox support by providing many features.

Leatrix Maps - Adds enhancements to the World Map. You can show dungeons and raids, travel points for same faction (this includes flight points, boat harbors, zeppelin towers and tram stations), travel points for the opposing faction, spirit healers and zone crossings (these are clickable arrows that indicate zone exit pathways).

TomTom - Your personal navigation assistant in World of Warcraft. This addon is very simple, but provides a nice set of functionality.A draggable coordinate display showing hundreth precision, this frame can be hidden or locked. WorldMap display of player and cursor coordinates, these displays can be hidden. The ability to set waypoints based on coordinates.

vMinimap(TBC) | vMinimap(Era) - My minimap addon. Has options for scale, position, hiding buttons, making the frame darker and hiding the LFG Eye unless in que in Classic Fresh. Be aware that the configuration is at the top of the lua file and must be done with a text editor, there is no in game config.

SpellBook Abridged - adds a "Rank Filter" to the spell book. Selecting the "Rank Filter" check box, you can toggle between displaying the entire spellbook and displaying only the highest rank of spells. Also has a basic Auto Up Rank function. When you learn a new spell that is a higher rank than one you know, this function will replace the previous rank of the spell in your action bars with the newest rank of the spell.

TinyPad - Simple but powerful notepad addon that's easy to use. I use this addon to store information I can look up quickly, like zone levels in the image. This addon has not been updated to TBC. Here is a fixed version TinyPad

What's Training? - adds a list of trainer spells to the spell book so you can see what is available, what is coming up and how much they will cost.

Threat Plates - Addon for customizing your nameplates. It offers a high amount of customization and tweaks to how your nameplates react and look.

GTFO - Provides an audible alert when you're standing in something you're not supposed to be standing in. In some cases, you'll be warned before you start taking damage. It's particularly useful for individuals that play with their spell graphics turned down or for those that just need a little extra help being told to GTFO. It also helps out with PvP when you can't tell who's casting the AOE.

Capping - A battleground timer mod with some extras. Current Features: Start Timers, Node Timers, Flag Respawn Timer, Flag Carrier Display with health percent. Port expiration timers/Queue wait timers/Scoreboard repositioning/Capture bar repositioning.

Questie - Shows where you can pickup quests, turn in quests, where to find mobs and items required for quests, etc. It puts colored markers on your map and minimap for quest objectives. Makes sounds on quest completion. Has an option for quest tracker. Puts icons on your map/minimap for NPC's like trainers so you don't need HandyNotes.

Leatrix Plus - Leatrix Plus makes the quest log double wide, adds quest levels and adds Wowhead links for quests.

Mob Search - Allows you to quickly find and target a specific mob in the game. Target a mob and hit Add, they will be assigned a raid icon. Each time you click the icon it will target the next closest mob matching that name.

vTooltip(TBC) | vTooltip(Era) - My tooltip addon built using Google Gemini. I moved the health bar to the top of the frame to look like an addon I liked, rTooltip by Zork. Other features include: class colored name and health bar, guild and guild rank, target of target, custom texture for health bar, quest difficulity coloring for mobs based on your level, scale and positioning, dark border or item colored borders, show names with no pvp titles. The quest information is being added by Questie. Be aware that the configuration is at the top of the lua file and must be done with a text editor, there is no in game config.

Leatrix Plus - Leatrix Plus makes the frame double wide.

Molinari - allows the player to hold down the Alt key and click on an item in their bags to disenchant or lockpick.

Profession Cooldown - tracks the cooldown of profession abilities across the characters on your account.

vUF(TBC) | vUF(Era) - My unit frame addon. Options for scale, position, class color health bars, turning off rest flashing and combat flashing, class icons instead of portraits, removing reaction color behind target name, hiding level, custom texture and making the frames darker. Be aware that the configuration is at the top of the lua file and must be done with a text editor, there is no in game config. Class Icons can be either turned on for all classes or for individual classes. I use Flat Player Icons for my Class Icons.

GnomishVendorShrinker - creates a simple, compact vendor frame. Recipes you do not yet know are highlighted in blue, Items you cannot use are highlighted in red. This addon has not been updated for TBC, here is a fixed version GnomishVendorShrinker

Reagent Filler - Takes the hassle out of restocking your reagents by offering automatic purchasing from vendors. Most class reagents are already added, additional items to purchase may be manually added.