This page will be for people to share useful addons

Deadly Boss Mods (DBM): http://wow.curse.com/downloads/wow-addons/details/deadly-boss-mods.aspx

This incredibly useful addon gives you warnings if you are standing in fire/black hole/acid to help your survivability. It also keeps track of boss health, cool downs, phases and other timers to help with healing or pew-pewing.

Ventrilo: http://www.ventrilo.com/download.php

While technically not an addon, this piece of software can help raids run much smoother. It allows for real-time voice communication between raid members. Use it to yell out target swaps, snobalds on your back or just to tell jokes.

Omen: http://wow.curse.com/downloads/wow-addons/details/omen-threat-meter.aspx

This addon takes the guess work out of who the big bad monster is going to be hitting. The tell-tell alarm lets you know when to pop agro reducing abilities if your leet AOE skills are too much for the tank to keep up, or if you happen to back into a patrol.

Recount: http://wow.curse.com/downloads/wow-addons/details/recount.aspx

While not a necessity to raid, this addon keeps track of damage taken, healing and damage done. This can help you pin-point areas of your DPS rotation or spec to improve (and how much the changes improve your DPS). It can also be helpful in determining what exactly made you go splat

Bagnon: http://wow.curse.com/downloads/wow-addons/details/bagnon.aspx

This addon helps organize your bag space as well as your bank. Adjustable frame sizes and all of the contents of your bags are searchable from any character.

Dominos: http://wow.curse.com/downloads/wow-addons/details/dominos.aspx

Tired of not having enough buttons on your screen at once? This is for you. Have up to 10 bars with adjustable sizes and opacity. Think of all the macros and items at your fingertips.

Gatherer: http://wow.curse.com/downloads/wow-addons/details/gatherer.aspx

Need some extra cash or just some mats for craftable items? Let this addon help you! With the ability to remember all nodes you have visited, or just use the wowhead database (http://www.wowinterface.com/downloads/info11707-GathererDBWoWHead.html) for all nodes visted by any player ever! This works great with the next addon...

Routes: http://www.wowinterface.com/downloads/info11707-GathererDBWoWHead.html

Use the information from gatherer to calculate optimum farming paths.

Altaholic: http://wow.curse.com/downloads/wow-addons/details/altoholic.aspx

Ever wonder when your alchemist alt can transmute? When the raid that your main was invited to is? How many badges your shaman has? This addon is for you. Keep track of your cooldowns, faction reputations, items in your mailboxes and much more.

Power auras: http://wow.curse.com/downloads/wow-addons/details/powerauras-classic.aspx

Keep missing your lock and load procs? How about hot streak? Use this addon to visually keep track of procs, buffs and debuffs.

Healbot: http://wow.curse.com/downloads/wow-addons/details/heal-bot-continued.aspx

You want to try out healing, but don't know where to start? The thought of mouse over macros scare you? Healbot can help. This provides a list of everyone in your party and raid, tracking debuffs and select buffs. Mouse clicks can be mapped to cast your favorite spells for quick use.

VuhDo: http://wow.curse.com/downloads/wow-addons/details/vuhdo.aspx

An alternative to healbot, this addon can also help healers react quicker. With more bells and whistles than healbot, it can be far more powerful, but does require more time to set-up.

Grid: http://wow.curse.com/downloads/wow-addons/details/grid.aspx

This unitframe addon can be useful for healers as well as DPS who like to know who is still alive and if they are in range of healers. While it needs a fair amount of set-up and auxillary addons (http://wow.curse.com/downloads/wow-addons/SearchResults.aspx?q=grid) this is a favorite of many raiders.

MikScrollingBattleText http://www.wowinterface.com/downloads/info5153-MikScrollingBattleText.html

Like seeing big numbers pop up on your screen, but hate the default blizzard combat text? This addon lets you control size, font, location and color of the combat text.