Our guild uses the following addons. These are required. Please have up-to-date for raiding.

Account security is not an excuse. Please get an authenticator to secure your account. It is extra security for your account and is $6.50 w/shipping.
http://us.blizzard.com/store/search.xml?rhtml=y&q=authenticator

Using the Curse Client(for macs too!) will be useful to manage your addons.
http://www.curse.com/client/

Required Addons

Omen2 - A threat meter
http://wow.curse.com/downloads/wow-addons/details/omen-threat-meter.aspx

Deadly Boss Mobs - Absolutely REQUIRED
http://wow.curse.com/downloads/wow-addons/details/deadly-boss-mods.aspx

Ventrilo Client - See in-game "Guild Information" for server and password information
http://www.ventrilo.com/download.php

Decursive- if your character has the ability to cleanse, purge, ect.
http://wow.curse.com/downloads/wow-addons/details/decursive.aspx

Strongly Recommended

HealBot (REQUIRED for Healers)(or some other frame-based like Grid+Clique)
http://wow.curse.com/downloads/wow-addons/details/heal-bot-continued.aspx

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

Elkano's BuffBars - Buff/Debuff Manager
http://wow.curse.com/downloads/wow-addons/details/elkbuffbars.aspx

Tiptac - Make your tooltip very useful
http://wow.curse.com/downloads/wow-addons/details/tip-tac.aspx

Please check our "Addons and UI" forum for more helpful addons.