Classtimer - tracks your own buffs and your debuffs on your target. Can be manually set to track item buffs such as trinkets aswell.
Tinytip - Places your mouseover tooltip on your cursor, which shows who your mouseover target is targetting and how much threat you have on em.
Power Auras - I'v just recently got into this addon and its great to set certain things shown you with animations which you can modify however you like. I use it to show external cd's on me such as guardian spirit, pain supression etc.
Omen threat meter - well this one is kinda obvious and mandatory to any raider.
Cork - Shouts not up
Grid/Clique - Compact way to see raid health, Clique for Vig and Intervene.
MagicTargets - Target's health
Obituary (with Xav/Premo hacks) - Last 3 hits that kill players
RaidBuffStatus - Raid buffs before combat
SatrinaBuffFrame - Replacement buff/debuff frames, very configurable, useful for so many things
TidyPlates_ThreatPlates - Easily see agro of all targets