Following is a compilation of addons (and links to them) that will probably make your time raiding (and also in general) easier.
The list will get updated each time we encounter new addons worth mentioning.
Threat
Used to calculate the threat of every raid member, to assist the tank/DPS to prevent pulling aggro.
Note that these addons work best when both the tank and the DPS have the same installed.
Damage Meters
All the following indicate various information about the raid - damage done/received, healing done…
Healing
Healbot - An improved layout to assist the healer, it also allows various configurations to make healing easier (keybindings, missing buffs etc…).
Grid - A compact layout to assist the healer by indicating health deficit, existing debuffs etc.
Decursing
This refers to addons that help you remove debuffs suchs as curses, magic, poison or diseases.
Class Specific
Hunter
Kharthus's Hunter Timers - This addon organizes the timers of all your abilities. From your autoshot timer (to help your shot rotation) to the Freeze Trap (to help you chain trap), it's a hunter lifesaver.