Well here's what my UI looks like. It's more than a little cluttered, but it works for me right now.
But what does all of this mean? Well, if I activate a couple of the add-ons I can show you what all of it is.
So what are my add-ons? I've got:
Bongos3 - my action bar add-on located at the bottom. I've got it full expanded to 120 buttons. Do I need all of them? No, but having the ability to put the items used for daily quests at the bottom has made doing dailys much easier. I use the same layout for each of my toons, but have character specific key bindings to make things easier. You'll notice I have my pet bar above my bag bar which is to the right above the main action bar. My menu bar is located in the center and my class bar which for my pally is my auras, is above and to the left. I've also got my stats bar to the bottom right. All of these bars are customizable in size and location. My action bar is one bar of 120 buttons. This can be broken up into multiple bars in whatever number of buttons you prefer and located all over your screen. It is highly customizable. You can also turn them off, but I keep the pet bar on for events like Chess in Kara or the Sunwell quest where you take over the ravagers.
Metahud - my range indicator. This is the add-on with the rounded bars to the left and right of me. The two green bars are my own health bar and the health bar of my target on the outside. The blue bar to the right is my mana bar. The mana bar of my target will appear on the right to the outside of my own. The options allow you to change the locations a little. The health/mana totals are also shown at the bottom of these bars including percentages. You'll note that I am casting Flash of Light with 0.7 seconds remaining shown above my mana bar. It's also shown just above my buttons, but that one is part of Bongos3.
VisualHeal - is located just below my menu bar and on top of my action bar. On my previous computer it was above the menu bar, just below my feet. I've forgotten how to move it. What does it do? The screenshot doesn't do it justice, but imagine my health being at 50%. When I begin casting a heal spell, it will show in a lighter green color the estimated health gain I will achieve from the spell. This works for any target.If you begin to overheal, the health bar will extend to the right past the bar length and show up red. Just a handy tool I like to use, but probably not necessary. It does not seem to take into account crits, because I've been at 20% and cast Holy Light on myself only to show it going up to around 50%, but then maxing my health out on a crit.
X-perl - is my Unit Frames add-on. If you're a healer and aren't using some kind of a unit frames add-on you are doing yourself and your raid a large injustice. I'm told that Pitbull is also an excellent add-on, but it is so highly customizable that I can't figure it out. You'll notice that my character frame has my mana/health with percentages, my buffs underneath me, and a casting bar. The target is next to me also with a lot of information. I also have target's target enabled, which I've moved to the right of the target. All of these can be moved to any location of the screen you prefer. One of the best advantages I've got enabled is the ability to see debuffs I can dispell. The party/raid member's bar will light up a light blue to show me they've been hit with something I can Cleanse. Sure helps being able to get that debuff off the tank who's about to take massive amounts of poison damage I can avert by dispelling it.
The party bars are underneath mine and I've located the raid bars around myself in the center of the screen. Does it get difficult to see sometimes? Yes, but I have to have them there in order to quickly see the health bars and also be able to see what is going on around me. A healer standing still and not noticing that rocks are falling, the blizzard is coming, etc. will die fast! The shaded roll area is where the items to be rolled for will appear. You'll also notice a cross right above my head. That's the Aggro alert. I always move it to the top of the screen, but it always seems to reset there. If you pull aggro a bright red Aggro alert will flash in it's location. Once you lose aggro it will turn green.
Recount - the damage meter. Mine is currently located to the right and turned to the Healing Done page. If you haven't used Recount it's much more than a damage meter. Besides showing heals, it'll show rezzes, dispells, etc. DO NOT KEEP THIS TURNED ON DURING A RAID!!! I only turn this on at periodic times of the raid like after boss fights or at the end. It gives great information and can show the last 5 fights or the entire total since joining the raid. I like to see who I'm healing the most and what spells I'm using on them. It'll give a breakdown and I can see how I'm progressing. BE VERY CAREFUL WHEN COMPARING YOURSELF AGAINST OTHERS!!! A little competition to motivate you to get better is one thing, competition to make sure you stay ahead of the others for the sake of being "the best" can get the raid killed! The ultimate goal is to keep the raid alive. Nothing else matters! If you do that, you could be "the worst" healer in the entire raid, but you did your job and can be proud of that!
Omen2 - the threat meter located under Recount. IF YOU DON'T HAVE A THREAT METER, QUIT NOW!!! No, seriously make sure you have a threat meter. If you're not sure how to use it, because it can be just a little confusing trying to figure it out in the heat of battle, make sure you talk to someone or do some research. They're really easy to understand once you actually know what you're looking at. I've recently acquired the Gavel of Naaru Blessings and noticed that my threat from heals has jumped up quite a bit, or at least I'm pulling a lot more aggro than I used to. More on that on a later post I'm working on.
I've got more add-ons that you can see, but primarily they are things like Gatherer, Atlasloot, Deadly Boss Mods (which is another that's required by my guild for raiding), Auctioneer, etc.
For healers I recommend HealPoints and RatingBuster. RatingBuster isn't healer specific, but it helps you know what you're getting from that +15 Intellect and +20 Stamina.