The following list contains addons that i tried personally and for me worked better than others. Some are required, because you cannot have another addon to work together with those, Omen is highly recommended over KTM (they can work together, probably with limitations), and X-Perl untiframes is just a matter of tastes (i use it with CT_RaidAssist for main tanks selection).

Please, before raiding, install the required addons. Note that without Group Calendar installed, you will not be able to attend any event we organize.

Group Calendar: (required)
description: a calendar to manage events in the guild and attendance, with no configuration required. It will be used to plan and organize raids, and heroic instances runs.
webpage: http://wow-en.curse-gaming.com/downloads/details/1958/

Omen (a threat meter): (required)
description: a threat meter like KTM, but lighter and with a better interface. It also assigns a different threat level for each mob with different names in the fight. It's updated more than once per day.
webpage: http://www.wowace.com/wiki/Omen
link to latest version: http://files.wowace.com/Omen/Omen.zip

Deadly Boss Mod (all Bosses mod): (highly recommended)
description: very usefull mod for boss fighting (a MUST have) capable of showing CD on bosses' abilities, and other fight related issues, triggering of boss events, etc.
webpage: http://www.deadlybossmods.com/

Natur Enemy Castbar: (highly recommended)
description: very usefull report tool on CD of your target abilities and debuffs, and CC countdown like polymorph, shackle undead, sap. You will always know when to resheep or reshackle, before it breaks ! It also shows the countdown on respawns in raid instances.
webpage: http://wow-en.curse-gaming.com/downloads/details/2786/

X-Perl UnitFrames:
description: all-in-one tool to manage raid/party/player/target/focus frames positioning and appearance, with some special support for raids
webpage: http://wow-en.curse-gaming.com/downloads/details/3416/