Let's talk about addons that are good for raiding! Share suggestions and the like.
Deadly Boss Mods (DBM)-
I feel this addon is the most helpful addon anyone, regardless of role, can have in a raid. It warns you of any special move the boss is about to execute. It warns you to move out any AoE you might not otherwise notice. It give you cooldown timers on bosses special abilities and things that hurt a lot. It gives enrage timers on bosses that enrage. It works in raids and instances. It's a great addon to have.
Recount-
This addon tracks things like DPS, damage done, healing done, dispells, HoT uptime, DoT uptime, and other useful things to know for you and everyone in your party. It shows all of this in a small graphical interface that you can place anywhere on the screen, resize, and hide. At also allows you to report the numbers in guild chat, party/raid chat, or whispers to a specific person.
Healbot-
As a healer, I love healbot. It allows you use any mouse click or mouse click/key combination assigned to any healing spell you can cast. I feel it's a faster and more efficient way to heal than just clicking on your spell bar. There are also other addons that do the same, Healbot is the one I personally use.
Omen-
This is an addon that tracks you threat. It's great to have to not pull of the tanks if you play a high threat-generating class. It allows you to know when to cool it for a few seconds so you don't pull threat off the tank and die.
PallyPower-
If you are a pally and you raid, this addon is your friend. At allows you to easily coordinate your buffs with other pallies in the group and avoids a 5-15 minute discussion of what buffs each pally is going to give.
Decursive-
If you are a mage, and there is a fight where you need to decurse, this addon makes everyone's life easier. All you have to do is click a name when it turns purple. You never have to deselect the boss and you lose little to no dps with this addon.
Ensidia Fails-
This addon is great for tracking your own and the rest of your party's screw-ups. It lets you know when someone 'fails'. Meaning they stood in an AoE, get hit by a tailswipe, a breath weapon, etc, etc. Just make sure that in the setting you have it set to show to you and only you. Showing the fail reports in raid chat is irritating to everyone.
Big Brother-
This one is great for raid leaders. No one else really needs it, but it's great for tracking a few things on your raid. It shows who has buffs, who had a buff-food on, and who has flasks on. It also shows a report whenever someone casts a misdirect or tricks of the trade, which is useful to know if you have a hunter or rogue who likes to pull aggro and die, yet never uses their threat abilities.
Headcount-
Another raid leader addon. It tracks who attended each raid, and how long they were there. It also keeps track of the loot people got. It automatically starts a new tracking page for each raid you enter. Until you delete it, it saves each raid in a list. Helpful for tacking attendance and keeping track of loot penalties.
Of course, those are just the ones I like. Other people use different things that do the same things, something completely different, or nothing at all.
A great tool for finding, installing, and keeping you addons updated is the Curse client. I loves it.
http://www.curse.com/