This is far from an exhaustive list -- it's always going to be a work in progress -- so if you know of addons I've missed that are ready to go for 5.0.4, pop a link in the comments or email me and I will add them to this list. There may well be many addons that are prepared for Mists' full launch in September but are not set up for 5.0.4 -- they're not what we're after here! If that's what you're looking for, though, there's a huge list of addons ready for Mists itself at Curse, and another at WoWInterface.
Please note that in the majority of cases, the addon page's live download will be updated at 5.0.4's launch, not several days in advance. Use your brain when downloading addons via these links; many of the authors who've contacted me have told me that their addons are ready to go but that they will not be uploading a release version to their live pages until patch day. This list of addons here represents those I have confirmed either through my own testing or directly from their authors that are ready for 5.0.4, and the links provided will be the links to download the addons on patch day or shortly before.
If there are addons you use that don't appear on this list, let me know and I will try to look into whether they're ready to go yet or if there is no news. I will do my level best to keep this list up to date and complete as we approach the launch of 5.0.4.
[I'd like to say a huge, huge thank-you to Bluspacecow for all their help with this!]
Action Bar Saver: allows you to setup different profiles for your action bars. Mainly this is for hybrid classes that want to be able to respec without spending 10-20 minutes setting their action bars up. 5.0.4 release will go live on Aug. 27. Download Action Bar Saver from WoWInterface.
Addon Control Panel: allows you to manage your addons in game, with an interface which looks similar to the blizzard addon manager. Not recently updated, but working nonetheless. Download Addon Control Panel from WoWAce.
Afflicted3: Afflicted is an enemy spell timer mod for PvP (mainly arenas). This is for people who just want useful information, such as an interrupt is on cool down and you're safe to cast a heal or that Blessing of Freedom is going to be back up for an enemy Paladin soon. 5.0.4 release will go live on Aug. 27. Download Afflicted3 from WoWInterface.
Altoholic: is an addon written for people who dedicate most of their time to leveling alts, and who want to have as much information as possible in one addon. The main feature of the addon is the search functionality which allows users to search their alts' bags or loot tables in an Auction-House-like frame. Download Altoholic from Curseforge.
Arkinventory Inventory/bag modification. Allows the user to create and assign categories to items. Download ArkInventory direct from developer website.
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. Download Auctionator from Curseforge.
Auctioneer: Developers are currently working on a release which will be compatible with WoW 5.0. We will hopefully see a preview release put up in the coming days. Download the Preview release from AuctioneerAddon.com.
Aura Frames: allows you to display any kind of buff or debuff on any given unit with highly customizable filtering and ordering functionality. Download Aura Frames from Curseforge.
Bagnon: merges all of your bags into three windows: inventory, bank and guild bank. Download Bagnon from Curseforge.
Bartender4 Action bar modification. Move, resize, reskin and reassign action bars. Download Bartender4 from WoWAce.
Bigwigs Boss timer addon. Modules for each boss with ability timers and the like. Download BigWigs from WoWAce.
BobBars an extremely lightweight, extremely basic modification of the default Actionbars.
Support configuration through the .lua file only. Download BobBars from WoWInterface.
Chinchilla Minimap Chinchilla Minimap. Alters the Blizzard minimap in a way to provide much nicer features and other niceties. Download Chinchilla from Curse.
Claw: addon for cat and bear druids that simplifies gameplay and maximizes DPS. Claw uses an advanced priority list developed by the people over at the SimulationCraft project and will calculate what ability would give you the highest DPS (and TPS) during the current combat situation. Also included are a combo point, energy, and global cooldown HUD. Download Claw from WoWInterface.
Clique Mouseover macro creator. Enables click-casting and hover-casting on unit frames, nameplates and more. Download Clique from WoWInterface.
CTMod: a custom user interface for World of Warcraft that adds tools ranging from the most basic functionality enhancements to advanced gameplay features, based on what the user needs. Download CTMod direct from the developer.
Deadly Boss Mods Boss ability timers and warnings. Beta and live versions not compatible with 5.0.4. Developer has confirmed that DBM is ready for 5.0.4; link will be added as soon as it is released. Follow developer MysticalOS on twitter for updates. Download DBM from Curseforge.
Dominos Action bar modification. Move, resize, reskin and reassign action bars. Download Dominos from WoWInterface.
Damn Achievement Spam: Compacts achievement messages. Download Damn Achievement Spam from WoWInterface.com.
ElvUI Once based on Tukui but has since gone through a complete overhaul of not only design but also file structure, UI features and the code itself. Designed to be more user friendly than Tukui by providing an elaborate and easy to use in-game GUI, which allows you to customize just about everything without having to reload your UI between changes. Download ElvUI from Tukui.org.
FreeUI: a minimalistic all-in-one UI replacement. FreeUI is in beta, and a 5.0.4 release is planned. Download FreeUI from WoWInterface.
FlyoutButtonCustom: Custom Flyout buttons similar to Blizzard buttons like traps and teleports. Now you can make your own buttons! Download FlyoutButtonCustom from Curseforge.
Gathermate2: Uses a database of nodes to provide a map overlay of node spawn points for all gathering tasks. Download Gathermate2 from WoWAce.
Grid Compact and highly configurable party/raid unit frame addon. Download Grid from WoWAce.
Healbot Allows for 20 key/mouse combinations to click-cast any beneficial spell. Current version seems to have some errors. A beta version is available from healbot.info.
Healium: provides a customizable, intuitive, compact, no-clutter interface optimized to make it easy to heal. Download Healium from Curseforge.
IceHUD IceHUD is a highly configurable and customizable HUD addon in the spirit of DHUD, MetaHUD, and others designed to keep your focus in the center of the screen. Download IceHUD from WoWInterface.
IfThen Allows you to create your own if-then statements that will dynamically change the function of a button. Intended for people who have a basic understanding of programming -- specifically, conditional if-then statements. Download IfThen from Curseforge.
JS Hunter Bar: Various UI modifications for hunters. A 5.0.4 version is already available, please note that it is unlikely to work with your 4.3.4 install. Download JS Hunter Bar from Curseforge.
LibResComm-1.0 A much more accurate system than Blizzard's built-in resurrection event and API. Download LibResComm from WoWInterface.
LUI: a next generation User Interface designed to replace the standard Blizzard UI. LUI is currently in development with a planned release for 5.0.4. Download LUI direct from the developers.
MogIt: Not updated since April 2012.
MikScrollingBattleText Makes it easier to see combat information by scrolling the information on the screen in separate, dynamically creatable scroll areas. Download MikScrollingBattleText from WoWInterface.
nUI: a user interface replacement for World of Warcraft. It is 100% standalone code designed to feel like Bliz meant the UI to look this way. Download nUI from WoWInterface.
Omen No information available.
OmniCC: an addon that adds text to items, spell and abilities that are on cooldown to indicate when they will be ready to use. Download OmniCC from Curseforge.
Perl Classic Unit Frames Replace the default player, pet, target, target of target, party, and party pet frames and add dedicated frames for many other units. Download Perl Classic Unit Frames from WoWInterface.
Postal: No news.
Power Auras Classic Designed to give visual or audible cues to the user in response to in-game events, such as gaining buffs or debuffs. Types of cues range from simple texture displays to stack counters, timers to timer bars, and are fully user-configurable. Can be animated or have sounds attached as well. Download Power Auras from WoWInterface.
Quartz: Quartz is a modular approach to a casting bar addon. Download Quartz from WoWAce.
Raven Customizable timer bar and icon configurations to monitor buffs, debuffs and cooldowns. Download Raven from WoWInterface.
Recount Meter measuring damage and various other raid performance statistics. Download Recount from WoWAce.
REPorter Battleground map giving a slim and readable overview of current battleground, allowing you to quickly send reports on any location and various other battleground factors. Download REPorter from WoWInterface.
RETabBinder Automatically changes your tab key binding to only target enemy players when in PvP zones and to target all enemies when not. Download RETabBinder from WoWInterface.
SetGuildRank Allows you to change the rank of multiple guildmembers in your party/raid with a single command. Download SetGuildRank from WoWInterface.
Shadowed Unit Frames Highly customizable but lightweight addon allowing complete unit frame personalization. 5.0.4 release will go live on Aug. 27. Download SUF from WoWInterface.
Shadowed Unit Frames Aura Indicators: Adds aura indicators to Shadowed Unit Frames, primarily this is for healers who want information such as what raid member is missing Mark of the Wild, or who has X debuff that they need to heal the raid member for quickly. 5.0.4 release will go live on Aug. 27. Download Shadowed Unit Frames Aura Indicators from WoWInterface.
Shadowed Unit Frames Trinkets: Allows you to quickly see if an unit has used its PVP trinket. Works with all units including party, focus, etc and will account for the 45 second shared cooldown that Will of the Forsaken sets on PVP trinkets. 5.0.4 release will go live on Aug. 27. Download Shadowed Unit Frames Trinkets from WoWInterface.
Skada Meter measuring damage and various other raid performance statistics. Download Skada from WoWInterface.
Skillet: a replacement for the default tradeskill and craft UI. Download Skillet from WoWAce.
SmartRes2 Uses LRC to display bars with rezzer, countdown, and target. When you bind an auto-rez key, SR2 automatically picks a viable target and starts the res cast, prioritizing other rezzers. Download SmartRes2 from WoWInterface.
TellMeWhen: provides visual, auditory, and textual notifications about cooldowns, buffs, and pretty much every other element of combat. Download TellMeWhen from Curseforge.
TidyPlates: enhances World of Warcraft's floating health bars -- commonly known as nameplates -- and provides a flexible theme system which can be used to create your own design. Download TidyPlates from Curseforge.
TidyPlates: Threat Plates: Adds threat component to TidyPlates. Download TidyPlates: ThreatPlates from Curseforge.
TipTop Tooltip addon that modifies the default tooltip rather than replacing it. Download TipTop from WoWInterface.
TitanPanel: Updated for MoP Beta. Not sure if this will work for 5.0.4. Download TitanPanel from WoWInterface.
TotemTimers Help managing totems, shields, and weapon buffs, showing additional info about active totems and timers displaying the cooldown of the most important spells and the number of Maelstrom Weapon stacks. Download TotemTimers from WoWInterface.
TradeSkillMaster Modular addon suite designed to help both casual gold makers as well as experienced auction house goblins streamline their gold-making processes including everything from crafting things to buying mats to snatch (dealfinding) lists to creating and canceling auctions. Download TSM from Curse.
Tukui Independently developed, clean user interface versatile enough to suit any role. Development focuses on simplicity, minimalism, and code elegance. Download Tukui from Tukui.org.
VuhDo Raid monitor similar to CTRaidAssist or Blizzard's built-in raid frames. Displays the health of raid members in form of clearly arranged bars. Healing spells or other actions can be asserted to mouse clicks on those bars. Download VuhDo from WoWInterface or download VuhDo from Curseforge.
Addons are what we do on Addon Spotlight. If you're new to mods, Addons 101 will walk you through the basics; see what other players are doing at Reader UI of the Week. If there's a mod you think Addon Spotlight should take a look at, email email@example.com.