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. 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.

BetterAddonList - BetterAddonList has the following enhancements: Search, Move the panel, Create sets that allow you to quickly enable or disable groups of addons. Including multiple sets in one allows you to create base sets of addons for enabling across multiple characters. Shift-click the check box to "protect" an addon, preventing it from getting disabled. Enabled addons with missing or disabled dependencies are shown in red. Load AddOn button is always shown for unloaded load on demand addons. See how much memory enabled addons are using.

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. It is a great replacement for the discontinued AdiBags. You can create custom categories.

WeakAuras - Allows the display of customizable graphics to indicate buffs, debuffs, and other relevant information. I use WeakAuras for my xp bar, buff reminders, unused talent point warnings, bag is almost full warnings and to let me know Find Minerals or Find Herbs is not active.
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

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.

Prat - a chat enhancement addon made up of modules which can be enabled that provide additional functionality. You can format how players names look in chat including their class color and level. There is an alt module that scans your guild log to find players alts, it then adds their main name in chat in parens after their current character name. It provides the ability to bind keys to chat channels, I hit G to type in guild chat.

ChatCleanerClassic - Customizable chat filter using required or banned words. Makes finding a group much easier.

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, Proxima Nova Condensed for chat and Lemon Milk for Nameplates. For directions on replacing game fonts go to the Fonts page.

MoveAny - 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.

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.

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. You can see Syndicator highlighted in red in the screenshot. I have a total of 42 Major Healing Potions spread between 3 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

I found this macro in the Blizzard UI forums. 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;

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 - 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. Other info I keep is the bags my characters have, gear from alts so I know if a drop is an upgrade, etc.

TrainerFilterFix - ensures that all trainer windows open with only the "Available" filter activated.

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.

Default Dark - If you prefer the default nameplate look this download has graphic files that darken the default nameplate. If you want to enhance the default nameplates try PhantomPlates.

DejaPRFader - Fades the party and raid manager frame (that bar on the left when you are in a party or raid). Fades in the party and raid frame manager on mouseover.

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.

BattleGroundEnemies - A lightweight Addon to show you all enemies, allies and important player-specific information in a battleground.

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.

Leatrix Plus - My quest log is handled by Leatrix Plus, it makes the 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.

QuestInfoTooltip - Adds quest info to the tooltips of vendorable items used as quest objectives. Will show if a quest is completed or incomplete on your current character. If your current character is intrinsically ineligible for a quest related to the item it will instead show the race/class/factions that the quest is available to.

vTooltip - A simple and lightweight addon that enhances your Tooltip. This is a modified ZaremTooltip. I moved the healthbar to the top like Zork did with rTooltip. The quest information is being added by Questie.

CloudyTradeSkill - Lightweight addon that improves default TradeSkill UI, Widen, Make Movable, Add profession tabs, Add bookmark recipes, Show recipe tooltip on mouseover, Show required item level

ClassicProfessionFilter - Allows searching known recipes and patterns in profession windows. Adds a check box to only show recipes that you have the materials to make. It also allows you to search materials which will bring up recipes that use an item, this is helpful for skills like cooking.

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 - 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 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. The download includes the class icons and texture.

WeWantGoldPaladins - This is an edited version of WeWantBlueShamans. It continues to change Shamans to blue but now it also changes the Paladin color from pink to gold.

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.

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.