I have never used any of these since I create my own UI, however I have taken various mods, graphics or fonts out of them. I have listed the most popular, ones I thought looked good, and some recommendations. Clicking the images will take you to the UI download page. The unit frame and bar addons used are listed below each screenshot.

UI Screenshot Site: WoW UI Gallery

WoW.com Reader UI of the Week

UI Compilation Threads:
Elitist Jerks
MMO Champion
Plusheal
Tankspot
Wowhead
Wowace

Arkive UI

Pitbull4, Dominos - Download

Arp UI

oUF, Bartender4 - Download
Resolutions: 1920x1080, 1680x1050, 1440x900, 1360x768

Barbol's UI

oUF, m_ActionBars - Download
Resolutions: scales from 1280x800 through 1920x1080

Bati's UI

STUF, Dominos - Download
Resolution: 1680x1050
WoW Insider Review
Bati's UI Grid Layout - WoW Insider Review of Layouts

Boda UI

Pitbull4, Bartender4 - Download
Resolution: 1920x1080

Caith UI

Pitbull4, Bartender4 - Download
Resolutions: 1920x1200

Calai UI

Pitbull4, Bartender4 - Download
Resolutions: 2560x1600

DreamingUI

STUF, Bartender4 - Download
Resolutions: 1400x900

Duffed UI (TukUI Base)

Custom, Custom - Download

Elv's UI (TukUI Base)

Custom with oUF core, Custom - Download
Resolutions: 1280 width and up
WoW Insider Review

Free UI

oUF, rActionBarStyler - Download

GrimUI

Custom, Macaroon - Download

gUI

Custom, Custom - Download

HissyUI

oUF, Custom - Download

KittyUI

Pitbull4, Bartender4 - Download
Resolution: 1920x1080

LUI

oUF, Bartender4 - Download
Resolutions: 1280 wide and up
WoW Insider Review

lumUI

oUF, rActionBarStyler - Download

Mono UI

oUF, m_ActionBars - Download
Resolutions: 1280 wide and up

Neav UI

oUF, nMainbar - Download

nUI

Custom, Custom - Download

OrkoUI

STUF, Dominos - Download
Resolutions: 1440x900, 1680x1050

Pat's UI (TukUI Base)

Custom with oUF core, Custom - Download
Resolutions: 1280 width and up

RealUI

Pitbull4, Bartender4 - Download
WoW Insider Review

Rekcinad UI

Pitbull4, Bartender4 - Download
Resolutions: 1920x1080 and 1680x1050

Roth UI zork

oUF, rActionBarStyler - Download

Shestak UI

Custom oUF, Custom - Download

Sik UI

SUF, Bartender4 - Download
Resolution: 1680x1050

Skaarj UI

oUF, m_ActionBars - Download

Skullflower's UI

Pitbull4, Dominos - Download

SpartanUI

oUF, Bartender4 - Download

SuperClassic UI

Custom, Custom - Download

Tarynn UI

SUF, Bartender4 - Download
Resolutions: 1680x1050

Thinktank UI

oUF, Dominos - Download
Resolutions: 1680x1050

Towelliee's Tank UI

SUF, Bartender4 - Download
Resolutions: 1920x1080

TukUI

Custom with oUF core, Custom - Download @ TukUI, Download @ Curse
Resolutions: 1280 width and up

Ultimate Shaman UI suicidalkatt

STUF, Dominos - Download
Resolutions: 1920x1200, 1920x1080, 1680x1050