Unit Frame Mods

Change Color & Texture - To change the colors to classcolors try StatusColor, to change the texture try StatusQuo both listed on the Art page.
Health Percentage
- To add health percentage to the default unit frames try TargetPercent.

bUnitFrames

Lightweight complete replacement for default unit frames. Includes: player, target, target of target, pet, pet target, focus, focus target & boss.
WoWInterface

Discord Unit Frames

Lozareth's highly configurable Discord Unit Frames.
WoWInterface

Galvin's Unit Bars

Displays bars that can be moved around on screen. Doesn't modify any of the existing UI.
Curse

oUF

Minimalistic unit frame framework. oUF does nothing by itself, you need to also install a layout. Most layouts have no in game setup but frames can be moved/resized using oUF_MoveableFrames. All other changes need to be done in the Lua code.
WoWInterface
oUF_MoveableFrames - is a nice plugin by the author of oUF which allows oUF frames to be moved and resized.  It greatly increases the flexibility of oUF for those not savy in lua.
Layouts - Curse, WoWInterface
Plugins: WoWInterface

PerlClassicUnitFrames

Perl Classic Unit Frames are a complete rewrite of the original mods by Perl. They replace the default Player, Pet, Target, Target of Target, Party, and Party Pet frames and add dedicated frames for many other units (Focus, Focus Target, Party Target, Target of Target of Target). It has been a high priority to maintain the look and feel or the original Perl mods while expanding functionality. The mod has been recoded for performance and is built around the Blizzard UI usage of events.
WoWInterface, Curse

PitBull4

PitBull is a Unit Frames addon that provides maximal customization and undergoes rigorous efficiency testing. It is created in a modular framework, which allows for mulitple authors to put their skills to work creating this great project. It has support for many different kinds of units: player, target, pet, pet's target, target of target, target of target of target, focus, focus' target, party, party pets, party targets, party pet targets, raid, raid pets, raid targets, main tanks, main tank targets, main assists, main assist targets, etc.
Curse
Info: FAQ, Guide, Forum Thread

Shadowed UnitFrames

Supports the basic units: player, pet, party, partypet, focus, focustarget, target, tot, totot, raid. The main focus in this mod is simplistic configuration, while maintaining flexibility that 99% of users care about, while you can't set that you want the health bar exactly 3.14 pixels outside of the health bar, you have options that you actually care about such as bar ordering, total frame height, bar spacing. All configuration can be done on multiple units at once through the global configuration, so if you want to change the height on player/target/party only, then set height for tot/totot/focus/focustarget you can do that easily. For the people who have to tweak every single option, you can enable advanced configuration which will show extra options. But, you do not need to do this by default, it's only for people who really want to tweak every single thing.
WoWInterface, Curse

Stuf

Stuf is a replacement to StellarUF. Stuf has the same default layout and feel of StellarUF, but Stuf is highly customizable. Do not expect much since this mod's purpose is to replace the Blizzard's main unit frames, not other unit frame mods.
WoWInterface, Curse

whoa UnitFrames

Modified Blizzard unitframe graphics with fat bars for player, target and focusframe, free positioning of player and targetframe via config.lua, health and manapoints with 1000 separators, optional simple health (rounded health and manapoints), you can see the percentage and the actual healthpoints together or just the actual healthpoints but you never see the maximum healthpoints, you can see the percentage just at healthbars (never for mana/energie/rage), threatindicator moved from the target to the playerframe, no charactername visible at the playerframe, healthbar classcolored for players and green-yellow-red-grey for npc's, you can change the size of buffs/debuffs (target).
WoWInterface

xPerl

A much enchanced version of Nymbia's Perl UnitFrames, and a complete replacement for Blizzard's default unit frames, including raid frames and raid tools, with many additions and improvements over the original Nymbia code.
Curse

HUDs

ArcHUD3

ArcHUD displays smooth arcs around your character in the middle of the screen to inform you about the health and power (mana, rage, ...) of you, your pet, and your target. In addition, it shows casts, combo points, holy power, soul shards, and a couple of other things. It fades when you are out of combat and at full health/power.
Curse

DHUD3

Player and target health and mana bars, casting bars, pet health and mana bars, druid mana bar in forms, bars with alternative power, spell circles with short player auras, target self auras and cooldowns, extensive target info, and much more.
Curse

IceHUD

This mod has alot of features and can replace your player and target unit frames and casting bar. I have mine set to be invisable when I dont have a target and im not in combat. Features: Player, target health and mana bars, Casting and mirror bars, Pet health and mana bars, Druid mana bar in forms, Extensive target info, Target of target display.
Config: /icehud
Curse, WoWInterface
Review: WoW Insider

miniHUD

Shows your health and target's health in small statusbars.
WoWInterface