Important Addons
ActionBarSaver (great for all those hard mode/pvp/etc respecs to save button layouts,
with everything hidden drag and drop isn't quite as easy even if you only need to change 2-3 spells)
AuldLangSyne
Autobar (Top Right)
Big Brother
BindPad (Used only for the occasional extra long macro)
BuffEnough (Good for seeing what buff you are missing)
Capping (PvP/WG Timers)
Chatter
Clique
CooldownButtons (small black bar on top left of grid shows spell cooldown buttons,
top bar to the left of autobar shows item/trinket cooldown buttons)
Dominos (bar on top right of grid for a few spells that change for specs/macros and range check spells,
vehicle bar shows above cast bar when in a vehicle, everything else hidden)
DrDamage
EavesDrop
FreeRefills (otherwise I forget to restock my arcane powder -_-)
Grid (most mages probably don't put this front and center, but I like to keep track of the raid and keep a similar layout for my other classes)
kgpanels
MacroSwap (hover on a macro button and it pops up your other macros, great for swapping fight specific macros)
MSBT
Opie (best addon ever, use it for armors/wards/teleports/portals/raid icons/conjuring)
Pitbull4
PowerAuras (only use for trinkets/heroism timers)
Quartz
QuickRepair
RatingBuster
SatrinaBuffFrame (love this mod, top left shows all buffs out of combat, in combat it filters out your typical long duration raid buffs/auras;
debuffs in top center, and then of course the large icons to the right of grid for hotstreak/brainfreeze, living bomb and polymorph)
Scorchio
SexyMap
Skada (set to show threat in combat and then switch to damage out of combat)
StatBlockCore (along with a few other StatBlock modules, used to create the fake fubar at the bottom)
Talented
TipTac
Xloot
I use a lot of addons and I tend to revamp my UI fairly often but I am really liking this layout.
Important information is easily visable and there is a lot of open space to see whats going on.