So let's talk about a few addons I consider essential for raiding.
Obviously I'm a big fan of Power Auras Classic for tracking cooldowns, procs and so on. (I also use GTFO, which is designed to work with Power Auras - it yells at you when you stand in stuff, which is pretty awesome. When I'm doing my mad kitty deeps sometimes I get lost in my rotation so it helps to have that little bit of extra "hey, stupid, gtfo" yelling at me.)
Some sort of boss mod is pretty much mandatory. I know most people use DBM - I used to use BigWigs (plus LittleWigs for 5 mans) but then after reading an article about it on No Stock UI, I switched to Deus Vox Encounters. I like the customization options and the boss health bar display. It provides arrows for certain encounters (pointing you towards or away from things like Lord Marrowgar's Bone Spikes and Malleable Goo) which I really appreciated until I started using HudMap.
Speaking of which, HudMap is another addon I consider essential for raiding. What it does is create HUD (heads up display) showing where you are in relationship to everyone else in the raid. An optional feature adds an arrow pointing to your target (assuming you're targeting another raid member) - this is GOLDEN on encounters like BQL where you need to run to someone to bite them or something. (I'm a kitty so I like to bite things. XD) It has encounter specific indicators as well, so you can see on the hud range indicators for shock vortexes, blood bolt whirl and similar mechanics. As far as I'm concerned, there's no replacement for it as a range indicator - even without the specific encounter mechanics it still works much better than anything else for seeing where you are in relationship to your fellow raiders.
(Note: The release version has had some issues with encounter visualizations for some users - the fix for me was updating to the alpha version, which you can download here. If you're using the Curse client, you should be able to just change your preferences to install the alpha for it.)
If you're playing a class that can cleanse, Decursive makes it much easier, though it doesn't do anything you can't do with mouseover macros or something similar. The downside to Decursive is that it lends itself to mindless cleansing which is dangerous on encounters where you want to control when and where people get cleansed. (No, I've never cleansed Mutated Infection on my priest when I wasn't supposed to. Not ever. >.>)
oRA3 is handy, especially for raid leading - it tracks cooldowns, checks durability, etc. I was using oRA2 but it's no longer in active development. I still haven't really explored all the features of the new one but so far I'm liking it.
And obviously some sort of threat meter (not so necessary for healing but essential for dps and tanking) - Omen is pretty much the standard for it, though some people really like Skada (which dual purposes as both a threat meter as well as damage meter). I played with it for a bit but it didn't quite have the display options I was looking for.
For raid, frames, I am a HUGE fan of Grid - yes, it takes some work to get it configured (though the most recent updates have dramatically improved the configuration interface) but once you have it set up, it shows you what you need and nothing more. For healing, you will probably also want GridStatusHots (it works not only for druid HoTs, but also spells like Prayer of Mending, PW: Shield, Beacon of Light, etc). GridStatusRaidIcons is essential for raid leading, esp if you feel the need to know if people are actually attacking the designated target and helpful for healing when you're dealing with a specific effect that DBM or other boss mods use raid icons to indicate. I also like GridManaBars and all the extra indicators I can get - text, icon or otherwise but I don't think they're necessary to make effective use of Grid. Most of the relevant Grid plugins can be found here. I will make a post later on explaining my Grid setup - for now, BobTurkey has written an excellent post on setting up Grid that I highly recommend (as well as his followup post).
What do you consider essential? Leave a comment here and let me know.