Pitbull is what I use for my unitframes. The reason I chose to use a unitframe mod is because the default UI doen't show a frame for your current focus, which is pretty damn lame. I also discovered that having the target of target of target is pretty damn useful for healing. Anywho, let me describe what's going on:

Top left: me
Top middle: current target
Top right: focus
Middleish: target of target
Bottom middleish: target of target of target. For healers this is useful to see if your target is actually getting hit (may have lost aggro).
Left: party members

All of that stuff is completely customizable. If you're an absolute control freak you can enable your party members' target. There is also a bunch of controls on setting up bar colors when your health gets low or whatever. By default I think it also uses this crazy 3D system for your portrait, but I found that distracting so I turned it off. I did find that I do need some sort of portrait on ToT and ToToT or else I would just kind of not be able to figure out WTF was going on. Apparently my mind processes pictures better than names *shrug*

Another nice feature is the ability to filter for certain buffs and debuffs based on your class or whatever. Very nice when you're in a big huge raid and the boss has like 40+ debuffs on him.

There are a bunch of other unitframes out there. ag_UnitFrames and xperl are the others I run across most often. They all pretty much do the same thing as far as I can tell. Pitbull and ag_UnitFrames are Ace2 mods by the way.

Ace 2 Updated	This program updates any mod in your interface folder that is made using the ACE2 framework (most are). It's really helpful in keeping all your addons up-to-date. Definitely the first thing to get when starting to download mods.
Outfitter	Outfitter allows you to create multiple outfits and equip them with a click of your mouse. You can layer outfits, create outfits optimized for a particular attribute (ie, fire resist or maximum defense), and have outfits which automatically equip.
Scrolling Combat Text	A fairly simple but very configurable mod that adds damage, heals, and events (dodge, parry, windfury, etc...) as scrolling text above you character model, much like what already happens above your target. This makes it so you do not have to watch (or use) your regular combat chat window and gives it a "Final Fantasy" feel.

Deadly Boss Mod	This mod gives you alerts for countless boss fights and is essentail to great raiding
KLH Threat Meter	KLHThreatMeter is like a damage meter, except it records your threat. By monitoring all the abilities you use and checking your talents, armour set bonuses and buffs, the addon can recreate the threat list of a mob. This makes tank transitions simple, and allows DPS to maximise their damage without pulling aggro.
Big Wigs	A boss alert mod. It consists of many individual "boss modules", mini addons that are designed to trigger alert messages and timer bars for one specific raid boss each. Modules are activated when you target or mouseover a Boss, or if any other BigWigs use in your raid does. In most cases only one module will be running at any given time.
PerfectRaid	PerfectRaid is a whole new look on raid frames, giving you as much information as possible in an easily recognizable format that's easy to place. Show as much or as little as you're interested in, and completely customize using the each options editor.
Damage Meters	DamageMeters monitors damage and healing done by and to yourself and other nearby players. It keeps a running total for each player and displays the information as a bars chart in a frame on the screen. It is perfect for real-time comparison of the various members of your party or raid. At any time the text results can be output as "says", to raid or party chat, or whispered to another player.

Lightheaded

LightHeaded is a very simple addon that displays quest information and comments from http://www.wowhead.com in game, eliminating the need to Alt-Tab when you get stuck on quest.

Bongos	Bongos is an action bar replacement designed to be both low memory, and easy to setup. It is not as feature rich as other bar mods, but should include the features most commonly used.
Bartender	Bartender3 is an Ace2 AddOn to provide more flexibility and functionality to the Action Buttons.

xPerl

A complete replacement for Blizzard's default unit frames

Bagnon

Bagnon is a group of addons that improve the way the player manages his or her inventory.

Necrosis	Necrosis is a mod to help Warlocks managing their stones, shards and demon summoning. No more, but no less...
Demon Trainer	Converts the merchant for warlock pets' books to a more friendly, trainer-style format.