Up until last night I used BigWigs, an alternative to DBM, but DBM shows you so much more information that I don't think I'll be going back to BigWigs any time soon. One especially key point in DBM's favor is that it will show you health bars for mobs that need to die at the same time, like Stalagg and Feugen or the trio of elementals on Freya. Still, BigWigs is almost as good if you want an Ace version or if you just hate DBM for whatever reason.
Download Deadly Boss Mods at Curse/WoWInterface (I'm not linking to their official site because it has gold ads)
Omen Threat Meter
This is pretty much the only threat meter anyone uses these days, as far as I know. Crucial for yelling at the DPS to back off, or the tanks to pick it up. And since raid leaders are often tanks themselves, it's a good way to monitor your own performance.
The default appearance is pretty terrible (parchment background, ugh), but it's relatively easy to configure.
As a note of interest, threat meters are now 100% accurate in WoW 3.0, since addons can access threat information directly from the game API - no more guesswork.
Download Omen at Curse/WoWInterface
Love them or hate them, a damage meter is a useful tool if you know how to use them right. Recount is my current favorite, although there are others around. You do need to take damage meters with a huge grain of salt, because many things can affect a player's performance (are they melee on a movement fight? did you put them on adds?). Still, information is good to have.
Recount is a fairly flexible and powerful damage meter. It records damage out, damage in, healing done, healing taken, dispells, resurrections, deaths, and more that I don't even remember right now. It can break down a player's damage, showing you how much comes from what spells (as pictured at right). It can list overall data, or data for any given fight. It can report to any channel you like. It works great for me, but again, there are many healing meters around; if you don't like Recount, find one that fits your needs.
Please don't put much stock in healing meters, though. They're next to useless.
Download Recount at Curse
Blizzard's raid frames are...well, I could politely call them "feature-light." From range to ready check markers, custom raid frames can give you much more vital information, and in a much more compact space.
I use Grid because I fell in love with it as a healer, but X-Perl is also extremely popular. Pitbull will also work and is very powerful and flexible, though it's labor-intensive to configure.
Download Grid at Curse/WoWInterface
Download X-Perl at Curse/WowInterface
Download Pitbull at Curse/WoWInterface
Honestly, I don't use this mod and don't plan to, but many people here at WoW.com think I'm crazy for that. It's basically a successor to the venerable CT_RaidAssist. It lets you do things like set up main tank frames, polls, and reagent/durability checks, and more. If that sounds like something you need, oRA2 is probably your mod.
Download oRA2 at Curse/WoWInterface
This was a new mod to me; I owe Chase big time for pointing it out. Acheron produces death reports. When somebody dies, you can right-click their unit frame and click the Acheron menu entry, and you'll see everything that happened to them in the six seconds (or so; configurable) prior to their death.
So whether it's because they stood in the fire or because they really weren't getting any heals, you can find out - and link it in whatever channel you want. Crucial. I used it at least half a dozen times last night in Ulduar.
Download Acheron at Curse/WoWInterface
That's about it as far as what I consider near-mandatory for leading a raid. Of course, you also need anything that you would want for helping you play your class, or helping you play in general (cooldown bars, a bag mod, and so forth). If your raid uses DKP, you might want a mod for that.
A chat mod like WIM could come in handy to manage all the whispers you will be deluged with from time to time, although for my money it takes up too much space; I just use Prat and a separate tab for whispers.
There are literally hundreds more mods out there that could make your life easier in raids. In this article I just tried to hit some major categories (and Acheron, which is a revelation to me). If you think I missed anything crucial, or if there is a much better mod in one of these major categories, let me know in the comments. Happy raiding!
Addons are what we do on Addon Spotlight. Power Auras Classic is another mod you might like in raids - it gives you pretty colored indicators for anything you like. And if you're a buff class, SmartBuff will make your life significantly easier. Remember, Addon Spotlight is fueled by viewers like you, so if you have a mod you think we should take a look at, drop us a line.