Tracks relevant buffs and debuffs on myself, my target and my focus in a concise highly visible format. The alternative is to weed out the irrelevant information from an ever shifting sea of buffs and debuffs in other places. Auracle is especially handy on my druid and other hybrid toons since I can pick and choose what to track in which forms and in which spec. Auracle also allows multiple effects to be tracked as one so if someone has applied a buff/debuff for me I don't have to myself. This is also handy for a focus crowd control target, you can track exactly when your crowd control will wear off and if you are working with someone else (i.e. PVP diminishing returns) you can track their crowd control in the same place.
NugComboBar Combo Point Tracker
Provides a way to display combo points for your current target front and center (or wherever else you like). Can also display holy power for paladins, shards for warlocks, evangelism for priests, and probably more I am forgetting. I prefer using it with the classic skin.
Outfitter Equipment Manager Replacement
The default equipment manager doesn't have great support for partial sets of gear. I have a few pieces of gear that go together for specific reasons for example a fishing set or a frost resist set that I have long since vendored. Outfitter allows me to equip those few pieces on top of other sets and just as quickly take them off again. Outfitter also provides a listing of all soulbound gear that isn't presently included in a set which allows you to quickly add new items to sets and sell unused items without searching through bags. Finally Outfitter extends the shift click equipment comparison to include same slot items from other sets so you can check if am item is a offspec upgrade without switching gear.
Quartz Cast Bar Replacement
Includes a representation of latency in the cast bar which allows me to begin casting my next spell as soon as possible rather than wait for the animation to complete. Little things like moving as soon as possible after mounting without canceling the cast become trivial. The increased size also does wonders for being able to recognize and react to enemy casters.
ArkInventory Bag/Bank Replacement
Combines all my bags into one giant bag which is sorted automatically by item type based on customizable rules; same goes for my bank. The benefit is that I don't waste any time sorting stuff in my bags, I open my bags and know exactly where my quest items are, where my greens to mail/disenchant are, etc. Another perk to ArkInventory is being able to search for an item by name to see which toon and/or which guild bank has some as well as getting item counts in tooltips.
Carbonite Map/Quest Tracker Replacement
Provides a wonderful resizable, movable, zoomable, translucent map frame to keep up while you are questing to show where you are in relation to quest objectives. Quite an improvement over the static map frame provided by blizzard. Also Carbonite will order active quests by proximity and track them all automatically which is convenient over the default hunt and peck quest system. I've also recently started using the resource node tracking in place of Gatherer so I can easily plan a route around a zone to hit all the ore and herbs.
Pitbull Unit/Raid Frame Replacement
The default unit frames can be configured to either display total health or a percentage, but not both at the same time. Percentages are important for many bosses while I prefer actual values otherwise. Pitbull allows me to customize my unit frames to display just the information I want. The default raid frames can't draw your attention to a player with a removable debuff aside from displaying an icon. I use Pitbull to highlight players with removable debuffs based on the debuff type. The other thing I don't like about the default unit frames is that the artwork around them takes up such a large percentage of the frame, Pitbull is very spartan so I can get more information that is easier to read in the same space or less.
Clique Unit Frame/Casting Enhancement
Clique does a number of things like allow you to create mouse shortcuts to click and cast spells directly onto unit/raid frames, but I use it for a simpler purpose; to select targets on mouse down rather than mouse up. I was surprised Pitbull didn't have this feature, but with the new default cast on button down feature it became pretty important to speed target selection to match spell casting to avoid casting spells on your previous target. Pretty big deal when healing. Now if only there was a mod that selected enemy mobs on mouse down so I don't keep casting interrupts on my previous target when trying to flip quickly and cast.
TotemTimers Shaman Totem Manager
The built in totem interface has a couple omissions which keeps me using TotemTimers. The main reason being that I can't switch my totem sets in combat. The default interface gives me 3 sets of totems, but I only ever use one and just flip totems as needed. It changes too often to remember and manage flipping between those sets in combat. I also like the countdown timers similar to the buff/debuff option which both TotemTimers and Pitbull provide, but I stick with TotemTimers for the combat flipping.
Fortress Data Broker Display
My new theory is to do away with a static bar and move my informational bits to various places around my screen where they are the most relevant. Fortress makes that possible as well as allowing me to create little bars/clusters wherever I want. There are so many nooks and crannies scattered throughout the default UI and I've taken a fancy to filling them with information I previously had lined up along the top with a data broker bar mod. I still use ChocolateBar on my laptop since space is at a premium on the smaller display.
DropTheCheapestThing Data Broker Plug In
Shift click drops the cheapest junk item in my inventory. The list of junk items defaults to grays, but can be expanded to include useless food, fish oil/scales, light feathers, etc. Really handy way to make space while questing or running a dungeon when you don't want to bother going out of your way to sell stuff. When visiting a vendor all junk items are automatically sold.
RepairBroker Data Broker Plug In
Shows my current gear condition as well as cost to repair. I've also configured it to automatically repair when I visit a a vendor that can do repairs, but only if I am exalted with their faction.
DeadlyBossMods Boss Fight Timers/Warnings
Provides timers for boss abilities as well as warnings when something important is happening, like I am standing in a big ol' fire. It also does some automated marking of persons of interest for some fights since I am often a raid leader or assist. I've most recently been impressed by the new proximity radar feature which replaced the /range list with a small map letting you see where people are in relation to you.
LushAdvancedLoot Need/Greed Loot Frame Enhancement
Displays how many people have rolled need next to the need button, same for the greed and disenchant buttons. Also displays a list of people that have not rolled next to the frame. All that information makes it much easier to decide what I should roll based on the situation. This project has since abandoned for some time, but the mod still works like a charm.
Skada Real-Time Combat Statistics Logging and Threat Meter
Tracks combat stats in real time on an overall or per fight basis. Really helpful for trying new things on a fight per fight basis and get a rough idea of what works best. I also use it quite often to figure out cause of death which is also tracked alongside death counts. As a raid leader it is also quite handy to have a rough idea of how your DPS compare in performance since that tends to come up in certain fights when balancing half and half or giving your top performers specific tasks. I am a recent convert from Recount/Broker_Recount since Skada is supposed to be more efficient, plus I was also able to get rid of Omen for tracking threat since Skada does that as well. Fewer, less resource intensive, mods means less time spent on the blue bar. I also use a couple extensions: SkadaHealAbsorbs for better healing info, and Recount like buttons with Skada Window Buttons.
Broker_Currency Data Broker Plug In
Shows the gold available on my current toon as well as my alts. It also shows PVE and PVP points and tokens as well as tracking how much gold I made/lost today, yesterday and the last week. Quite handy fo figuring out which toons to rob when I am looking to make a large purchase.
Wow Instant Messenger Whisper Frames
Opens a new frame for each whisper conversion when out of combat. Trying to keep guild chat straight from multiple whispers and not accidentally reply to the wrong person/channel isn't easy without an add-on. The default chat frame is good for splitting large channels apart, but whispers are rather fluid and are better suited to separate windows.
DoubleWideTradeskills Profession Frame Enhancement
Doubles the width of my trade skill frame making it a lot easier to find things with less scrolling. I go from beign able to look at like four recipes at a time to 14. Recipes on one side and details on the other. Basically makes it look like the quest frame.
ProfessionsVault Known/Learnable Recipe Coloring/Tooltip Extension
Shows which toons can learn as recipe, or could learn once their skill is higher, right in the tooltip. Very helpful when looking through a guild bank, auction house or even a random vendor to see if any recipes are useful in the sense that I don't have them yet. It also colors the icons to make it even more obvious. Best of all the profession scanning on each toon is instantaneous.
CastBy Buff/Debuff Provider Identifier
Shows me who buffed/debuffed what right in the tooltip. I know who dotted that crowd control target. I know who left aspect of the pack on. I know which pally did one buff so I can ask someone else to do the other. Surprisingly handy, I am amazed this isn't part of the default UI. This project has since abandoned for some time, but the mod still works like a charm.
I am not an auction buff, in fact I am quite the pack rat and can't be bothered to sell anything half the time. Occasionally I identify some surplus item that I will likely never use and will go to auction it off. I used to use Auctioneer which while very feature rich has become extremely bloated for what I actually used it for. Auctionator provides everything I used Auctioneer for in a smaller footprint.
kRestack Bag/Bank Space Reclaimer
ArkInventory has a button to restack items to reclaim bag space, but as far as I know it still doesn't have this capability to move items from generic bags into profession specific bags when possible. This add-on also automatically does the restacking where as I have to hit a button in ArkInventory.
Capping Battleground Timers
Shows timers for battleground resources and/or nodes as well as win timers when applicable. Also shows some extra information like flag carrier names and health in Warsong Gulch and Eye of the Storm. Oh, and it shows the ever important Tol Barad timer and who is currently in control.
Displays a line in my chat window with the name and time someone pings the minimap. I must admit I get more use out of it poking fun at those who repeatedly ping the map than anything useful, but it has been helpful in a raid or two.
Tidy Plates Name Plate Enhancement
More importantly Threat Plates is what I actually grabbed this for. I've relied on just watching mobs with an eagle eye for so long I never knew what I was missing. Keeping an eye on a large AOE group is so easy when any uncontrolled mobs are highlighted. I also plan to make use of the name plate debuff tracker to help track my debuffs on multiple targets. Should come in handy for my affliction lock as well as lacerate stacks on my bear.
Overachiever Achievement Tools and Tweaks
This add-on tells me when I need to kill a critter, love a critter or eat/drink something for an achievement. I'm a passive aggressive achievement whore. I can't be bothered to actively pursue an achievement, but if I happen to be there an an add-on can tell me in a tooltip to do something, then sure. Overachiever also allows me to move my achievement window around which is handy sometimes.
NeedyGreedy Data Broker Plug In
Shows who rolled on the past few items, what they rolled, if they needed, greeded or passed, and who won. It also displays icons with tooltips for the items that dropped.
LargerMacroIconSelection Macro Icon Selection Enhancement
Makes the itty bitty window where you give your macro a name and an icon much much bigger so you can actually find a specific icon without going crazy. I usually don't go looking for icons, but once in a while I do and this is really nice.
LoggerHead Automated Combat Log Enabler
I often look to online analysis tools such as World of Logs to compare progress over time and get a much more detailed look than a tool like Recount can provide. The problem is that to use online tools I have to remember to enable combat logging to write the combat log data to a text file. This mod asks each time you enter a new instance if it should be logged and then takes care of it automatically; otherwise I am prone to forget.