CT RA is the greatest thing since porn. So helpful that most large guilds require use of this mod. I never raid without it.
Decursive
Decursive will bind all your single target non-buff spells into one button. Works with; Priests' Dispell, Shamans' Purge, Paladin's Cleanse, Mages' Lesser Curse, and Druid's Poison/Curse. For Example: Priests' Dispell will automaticly remove negative effects from you, your group, your raid group, and then pets (only if the Debuff is harmfull to the player IE a rogue with Ignite mana will be over looked).
Atlas
Atlas is a great mod for learning instances inside and out. Shows bosses, rare spawns, and quest item locations.
Loot Link
Loot Link is a cool mod for linking items into chat without having to physically see the item.
Titan Panel
Titan Panel is for the lazy WoW player. Overhead bar shows XP gained per kill/hour, estimated time until next level, honor gained and estimated time until next rank, gold, inventory space available, ammo available, coordinates of your character, and a riding mod that changes out your "Carrot on a stick" & Boots & Gloves w/ riding skill enchants when you summon your mount, and does the inverse when you dismount (if not in combat).
Damage Meter
Damage Meter Mod is pretty cool to see who is the better player, or had the better gear. Tracks all damage done by players of your raid and allows you to spam the damage into raid chat so everyone can see who kicks more ass.
Mortal Kombat Mod
Need I say more? Click the link and "Choose Your Destiny".
Scrolling Combat Text
SCT is a helpful addon that displays damage/negative effects over your character and allows "sticky crits" for when you crit and want to SS it before it disappears.
WOW calender mod
This is a very useful mod that allows everyone to see upcoming guild events and sign up for them. Guild members may also set up guild events (subject to council approval) of their own with this mod as well.