I pretty much downloaded the ui in parts. Below is a list of the addons.

x-Perl Unit Frames Awesome UF's with party cast bars and very customizable.
Meta-hud This is the hud you see in the middle around my toon.
Totem Timers Nice tool for keeping track of totem timers and totem procs.
Fubar Everyone knows fubar, awesome addons
Bartender The best bar addon iv ever used. I have it setup in the bottom left with a button setup that mirrors my Nostromo n52 gamepad.
Simple-minimap The most customizable mini map addon you can get, skin-it or make it go away, very nice.
NE-CB Natur enemy cast bars, very nice cast bar addons.
Qaurtz Awesome cast bar addons which has a cast bar for everything you could need, even autoattacks, and target/party bars.
HItsmode This is that cool chatbox you see at bottom right, a must have for people who like to see whats going on in the combat log, very nice and customizable. I highly recommend it.
Damage meters, and Omen you can also see open

Also some addons of interest you cant see but are valuable in mutli-boxing.
Auto-repair, auto-sell, auto-group, two-box, and quest helper and lightheaded. I use more then this but these I feel are the essential.

As you can see I like to keep my UI kinda clean and tidy. I