Auramastery - makes icons/timers appear if certain checks go through. For instance when you use a skill, show the cooldown, or show when you/enemy has/doesnt have a certain (de)buff. Incredibly valuable on certain raid bosses.
Ayth quest - draws lines to quest objectives, quest givers, challenges, that kinda stuff
Mailhelper - one click button to loot and delete all lootmail
Optiplates - brilliant nameplates addon, lots of customization with threat alert and cleanse indicators, size scaling for nameplates based on distance, lots of other useful stuff. Also better optimized than the stock nameplates i believe, so more FPS.
Vince builds - allows you to save different skill and gear loadouts that you can link together. For instance you can have 3 different DPS specs linked to your DPS gear and 2 healspecs linked to your healgear and switch between them in seconds.
Skillswitch - adds small buttons that allows you to change a single skill really quickly, great in combination with vince builds; have 1 or 2 general DPS specs that you can quickly finetune with skillswitch.
TapThat - does all those pesky minigames for you; press F at regular intervals, spams F, does the simon says minigame etc. Doesnt do CC breaking so combat isn't affected.
Virags autocircuit/autocoordinate - makes crafting a breeze, allows you to craft large batches of the same item. Makes crafting dailies incredibly quick.
Viragsmultibuyer - allows you to buy more than 1 item at the same time from a vendor.
WYBMNRedux - housing addon that makes it very easy to add neighbors via a filtered list (IE only housing plots with a thicket and 50/50 sharing). Also adds a 1click button to the next house. Brilliant if you farm housing plugs which can earn you quite a bit of cash.
Got plenty more installed, but those are the most useful for most players i think.