I have listed the most popular, ones I thought looked good, and some recommendations. Clicking the images will load a larger view. I have listed unit frame and bar addons along with posted resolutions below each screenshot.

UI Compilation Threads:
MMO Champion
Plusheal
Tankspot

AffinitiUI

Download: WoWInterface

Aftermathh UI

Download: WoWInterface

AsphxiaUI

TukUI Base - Download: TukUI

DerpyUI

Download: WoWInterface

Dargan's ElvUI

ElvUI Base - Download: WoWInterface

DecUI

ElvUI Base - Download: WoWInterface

DuffedUI

TukUI Base - Download: TukUI

ElenIU

Download: WoWInterface

ElvUI

Download: TukUI

FictionalUI

Download: WoWInterface

Free UI

Download: WoWInterface

GrimUI

Download: WoWInterface

gUI

Download: Curse

LeilaUI

Download: WoWInterface

LUI

Download: WoWInterface
WoW Insider Review

MayronUI Gen2

Download: WoWInterface

Mono UI

Download: WoWInterface

Neav UI

Download: WoWInterface

nUI

Download: WoWInterface, Curse

Qulight UI

Download: WoWInterface

Quse UI

Download: WoWInterface

RealUI

Download: WoWInterface
WoW Insider Review

Roth UI zork

Download: WoWInterface

Ruffio UI Classic

Download: WoWInterface

Shadowmage's UI

Download: WoWInterface

Shestak UI

Download: WoWInterface

Sik UI

Download: WoWInterface

Skaarj UI

Download: WoWInterface

Skullflower's UI

Elv UI Base - Download: WoWInterface

SpartanUI

Download: WoWInterface

Tarynn UI

Download: WoWInterface

Towelliee's Tank UI

Download: WoWInterface

TukUI

Download: TukUI

Ultimate Shaman UI

Download: WoWInterface

ViksUI

Download: WoWInterface

VxtUI

Download: WoWInterface

Wullie's UI

Download: WoWInterface