Be ready for the terrifying addon list of doom...
!!!ZOOM!!! - makes the loading screen go by a little faster by doing something I can't remember
AchScreenshotter
Addon Control Panel
Auctionator - a lot simplier than Auctioneer
BlizzMove - allows me to move the various windows around as I wish
ButtonFacade
Chinchilla Minimap - I like my minimap square and with as few buttons as possible
Clique - allows me to make some nice mouseover click macros. For some reason the normal macros can't cast on mouseover if the macro is assigned for a mouse button, so Clique takes care of that.
Deadly Boss Mods
Dominos
eAlign - helps me align my UI just right
EveryQuest - to help me find the few quests I'm still missing in Azeroth
GearScore - while I find the thing misleading and overrated, I like being able to see right away what kind of people I might be dealing with.
Grid
HandyNotes - allows some addons to make helpful notes on my map
HighUI - takes the graphics even further than WoW's own settings allow. Also makes it a lot easier to manage the level of my graphics without going into the display menu
InlineAura - displays aura information inside action buttons
Mapster - gives me some control over the size and placement of the World Map
Moncai Quest Tracker - some tweaks to the Quest Tracker
OmniCC - shows all the cooldowns in numbers over the skills
OneBag3
Overachiever
PetEmote
Postal - makes opening and sending mail a lot easier
Power Auras - mostly I use this only on my mage to see when my free Frostfire or Pyroblast proc etc
Prat
Quartz - better casting bars
Reddrake - informs me if I'm missing any Glory of the Hero achievements in the instance I'm in
Shadowed Unit Frames - my favourite UF addon by far, SUF is both simple and versatile
SilverDragon
SimpleBuffBars
Simple Treat Meter - replaced Omen with this, shows me the amount of aggro I have on my current target. Is just a black bar filling up, which makes this much easier to find room for.
Skada - replaced Recount
SmartMount - all my mounts and falling speed reductions brought to a single keybind <3
SpamMeNot - saves me from all the gold spammers
Spy - lists all enemy players nearby. I play mostly on a PVP server, so it comes handy.
TipTac - prettifies my tooltips
Tooltip Wrangler - controls the position of my tooltips
UpRank - makes sure all my skills at their highest ranks
VendorReward - adds a handy Vendor button the quest turn-in window that automatically picks the most expensive reward.
ViperNotify - lets me know if I've forgotten to switch out of AotV
XLoot - some nice tweaks to the loot window
Might upload a SS to show what goes where later. My UI really isn't as cluttered as the above list might suggest - most of them are just to achieve the clear look I'm after.
It's missing some stuff, like DBM but.. Meh. :P You get the idea. I keep the lower right corner empty, as that's where all the MSN and Skype info pops up.