Deadly Boss Mods - An addon that shows timers and makes announcements for certain specific events that happen during boss fights - usually things that need interrupting, stacking on, running away from, etc.
Skada Damage Meter - Allows you to view output, interrupts, etc, during/after a fight.
Decursive - A compact UI that makes it easy to dispel raid members.
Angry Assignments - A useful tool where you can set tactics for specific boss fights that you can have on your screen. Used as a raid leader you can post those tactics to chat.
Auto Combat Logger - Set to automatically start combat logging when you run specific raids.
Identity-2 - Add an extra "nickname" when you talk in guild/raid/party chat, etc. Useful when you're on an alt and you want people to know it's you!