Here's my current UI, and I've made some changes from the last one. Here are the addons I use
Grid - provides a very neat and collected look to a party/raid
SexyMap - changes the minimap skin, and contains several textures and animations
DetachedMiniButtons - allows you to move the addon buttons (and server time box) off of the minimap (could not get GEM3 to stay in a specified location on the screen. When I clicked to bring up the calendar, it moved back to the minimap, so I just autohide it).
Bartender - makes all action bars movable and scalable
Itemrack - allows you to quickly switch gear, very useful with the new dual specs
Icehud - shows arc bars around the player, providing health and mana/rage/energy information, as well as target information
PvPLog - keeps a log of any pvp activities
Healbot - healing addon that makes healing in groups/raids more effective
Omen - threat meter
Recount - dps meter
Atlas - gives instance maps, as well as other various maps
Atlasloot - shows loot for various instances, factions, events, etc
Atlasquest - provides quest information for each instance currently viewed in atlas
Cartographer - makes the world map scalable, provides instance maps, and changes party icons
Chatsounds - gives sounds for when people type messages
Deadlybossmods - gives various alerts to boss fights
Decursive - allows for one-click decursing on party and raid members
GEM3 - allows for creation and managing of guild events
MailGet - quickly retrieve mail loot from your mailbox
WIM - moves whispers into its own box seperate from the chatbox
