Deadly Boss Mods
DBM is useful in a way that it alerts you on when a Boss begins to cast certain spells or use certain skills, so that you know when to move, counter, stop damage, run away, etc..
AtlasLoot Enhanced is an UI mod allowing for loot tables of bosses to be browsed whenever needed within the game.
Recount allows you to view many statistics about your Party / Raid Group such as Overall Healing, Damage and DPS in a detailed fashion.
Omen Threat Meter
Omen is good for Tanks and High DPS / Healing players to show what percentage every player's threat is at according to what Mob is selected.
This addon explains itself in the name. In addition to showing you exact map points of Questing "Go-Tos", it will also show you the direction you need to travel, and how far away you are from your destination. Quest Helper also allows you to share and see other party members quests.
OmniCC is an addon that adds text to items/spells/abilities that are on cooldown to indicate when they’ll be ready for use.
Gladius adds enemy unit frames to arenas for easier targeting and focusing. It is highly configurable and you can disable most features of this addon.
HealBot is a massively supportive addon for healers
More addons to come... Most of them are for 4.1 patch, but i will search for 3.3.5 version and post it...