Add-ons have become more a stock part of most player's UI than the blizzard default. Why is that? Well, frankly, blizz's version is pretty basic and not as helpful as it could be with a lot of things. I use the add-on's below to help with that.
Advanced Trade Skill Window: This is my newest add-on and one I wish I'd discovered previously. Want to see only the glyphs you can make for paladins? How about only leather items that will get you a skill point? No more scrolling through seemingly endless lists of recipes to get to the one I want. I also like the ability to queue up items to create later. Although I haven't used it, I could see this being very handy.
Auctioneer: This seems to be a standard for just about anyone serious about the AH. I am not what you would call serious, but I do like to recognize a good bargain. Obviously, it has some weaknesses, and isn't easy to use to it's full potential (like anything else, it's a tool, only as good as it's user). Yes, on patch day I bout some level 70 epic gems because it said they were a good deal, and I bought them out before realizing they were essentially useless. oh well.
Cartographer: This add-on shows the whole map. I semi-understand why blizz doesn't do this (all the way back to the first warcraft, if you havent been there, you don't get to see it), but come on! In the age of instant info, statgey guides and complete maps on blizzard's own website, the least they could do is make this a standard feature.
Deadly Boss Mods: I picked up this add-on for one reason: Dark Smash. I was tired of getting crushed in UK. It has become very handy knowing when bosses are about to unleash the nasty. Not that it has kept me from dying, I just know ahead of time, lol.
Dominos: I love this mod more than life itself with one exception, but I'll get to that. One bar of icons just isn't enough, especially past say level 30 when you have a tendency to use 8-10 abilities constantly and many more you use conditionally. I normally have about 4-5 bars on my screen at once, and put temporary quest items there too, just to make it easier to use them, its great. My only complaint is with warriors and druids. The forms you're not using are on bars that you can switch out, but they have to be someplace, even if you can't see them. This can sometimes get in the way and is pretty annoying. I've also unknowingly removed icons. I know you can lock the bars, but I like being able to switch things around if necessary.
Gatherer: This addon marks resource nodes on your mini map that you've gathered from. Its convenient for me for when I'm trying to decide to take a direct route, or a more roundabout one which includes some nice gathering.
Healbot: This is another one that is new to me. I had originally thought it was just quick mouseover healing, I was wrong. It also includes unit frames which makes healing a lot easier. Now I can more easily see who's taking damage and quickly apply whichever heals are necessary.
Omen: Who wiped the group? This addon tells you where you're at with aggro, and it works with pets too. You get a nice flash of red and sound telling you that you pulled aggro. When tanking, it lets me know where everyone is with the caveat it is mob specific, so if they're DPSing outside kill order, you'd have no way of knowing (gee, if only there was a way you could mark mobs to tell players which to focus on in what order).
Pally Power: (amidst my protests) I don't like this addon, and it gets in my way 95% of the time. I mostly solo things, and the last thing I need is more useless crap on my screen. However, our guild has a ton of pallys (we can have up to 6 in a 10 man group, 2 prot, 2 ret, 2 holy, more normally we'll have half that), and managing all the buffs and auras can be a pain, so this is "required" when raiding.
Quest Helper: This mod has become so widespread, blizz has included many of it's elements into their default UI. Love the quest tracking, love the routing, pretty much love everything about it.
Titan Panel: This is one of the first addons I got, and still one of my favorites. It gives you a little bar at the top of the screen customizable with all kinds of useful information. I currently have coordinates, bagspace(used/total), gold (which if you hover will give you total on that server not in the mail), and I'm sure a few things I'm drawing a blank on at present. Again, super customizable and convenient, I REALLY miss this when it's not around.
Which one is my favorite? Well, if I could only use one, it would be dominos, I absolutely can't play without it now. All the others do what addons are supposed to, make life just a lil easier in game.
Mid Afternoon Quickie
9 months ago