UPDATED AGAIN! (by Straif)
It is highly recommeded that everyone download and regularly use WoW Matrix to keep their add-ons up to date. This is a seperate program that should be run prior to Warcraft (WoW can be launched directly from WoW Matrix) and will check, and if necessary update, all of your add-ons. It's list of add-ons is immense and includes all of the ones mentioned below (with the exception of Vent and voice overlay which are seperate programs themselves) as well as almost any other one you can think of.

And for those users who are currently using the WoW ACE updater, WoW Matrix can update all of the ACE add-ons as well as a whole lot more.

UPDATED!
These are the mandatory plugins that need to be running. You MUST be
running all of these plugins and Vent if you want to run any raid instance

Mandatory for All Classes
1) Omen
- Threat meter, shows your threat to everyone and you everyone's threat.
Quite revealing isn't it? If you do not have this plugin, you will not run.

2) Ventrilo(Client)
- Ventrilo in game chat. This is a program that runs outside of WOW.

3) Voice Overlay
- Voice Overlay for Ventrilo and TeamSpeak. This allows you to see who is talking while in game.
Not necessarily required but very useful when using an external game chat program
such as Vent or TS.

4) Deadly Boss Mods or Big Wigs (preferred)
- This will give you warnings when a boss will preform their 'special' moves. When Moroes will diassapear and garrote or if you are too close together in specific fights.

Class Specific
4) Decursive
- Debuff purging addon, refer to post #3 for a more detailed explaination

5) PallyPower
- Paladin specific plugin for buffs, refer to post #3 for a more detailed explaination

The zip files need to be unziped into World of Warcraft\Interface\AddOns directory.
Once you have done this, you can start WoW and the plugins will be enabled.[/b]

If you have the ability to remove debuffs such as curses, poisons, magical affects, etc... Decursive is pretty much a requirement.

It's small, simple to use, and even auto-configures itself for your classes specific abilities.

What makes this add-on so useful is it does not require you to change target to remove a debuff. Each member of you party is represented by a small square and when afflicted by a debuff that you can remove the square will light up. You simply click the square and the appropriate spell is cast to remove the affliction. Your target will never change so the only interruption to your casting, shooting or melee is the fraction of a second your debuff removal spell takes to cast.

It is also a part of the ACE2 family of add-ons so if you have the ACE2 updater installed it will keep Decursive up to date.

For any Paladins in the guild interested in running raids Pally Power is a must have.

The hassle of trying to synchronize 2 or more Paladins and their various blessings can easily eat up precious minutes after each wipe or partial wipe. Pally Power allows the raid's head Pally to set each Paladin's blessings according to class type therefore bypassing the mass confusion that is bound to happen.

It also allows each Paladin to see the time remaining on each blessing they have made as well as see if there are any raid members they have missed.

And for those Paladins who do not do a lot of raiding yet, it is still a useful add-on to have even for small groups or soloing.