And I mean everyone. Whether you play PvP or PvE, craft, run dailies, whatever. These addons or ones like them will make your game play that much smoother. Every now and then I clear all my addons and reintegrate them one at a time. These are the ones that are always present:
1. Addon Control Panel
It's most basic function is to add an Addons button to the game menu. This allow you to turn on and off addons with only a reload. No more logging in and out repeatedly.
2. Bad Boy: Anti Spam Automatic Reporter
Since installing this addon, I've had no problems with gold sellers and phishing attacks. I don't need that garbage polluting my chat.
You don't need to be any kind of AH maven to know that WoW's default AH UI is woefully deficient. It'd be nice if it got a
major re-haul in 5.0 like raid frames did in 4.0, but I'm not holding my
breath. Check out Gold Capped's article to see how much better this addon is.
In the same vein as Auctionator, Postal invigorates and gives function to a part of the interface where the default UI is lacking.
SellJunk adds the convenience of selling all grey items with a single button press. It's a minor quality of life improvement that everyone can appreciate.
I also use two other addons that I consider good for a mention, but not necessary for every player. Minimap Button Frame gathers up all the stray buttons addons add to your map into a neat window. Reflux manages addon profiles. Together, these two make setting up and managing a UI smoother.