[Jelinda]

I've been looking for an addon that gives an obvious warning, either auditory or visual, when someone in my group/raid gets aggro. There are lots of addons that warn you when you GET aggro, but as a tank I want a warning when I LOSE aggro. I don't want to stare at my healbot (or whatever you use) for 40 minutes, just to be ready to click my emergency macro in the 41st minute when someone's bar gets the red aggro framing. I just want a big obvious warning so I can focus only on the fight as long as there's no aggro change.

[~Luffie~]

I don't think there's an addon for this. I've been using Xperl's big "AGGRO" warning for this and I have it placed in the center of my screen. It's red when you have aggro and changes to green/disappears when you lose aggro.

[Squirrelnut]

Closest thing that I have found useful is the 'Alof' nameplate addon. Basically the mobs nameplate glows red when you have aggro and doesn't if you don't.
Aloft - Addons - Curse

[Molohk]

Yup, Aloft works great, the nameplates glow yellow if you're close to losing aggro, and they stop glowing when you lose aggro.

Unit frame addons like Pitbull and Grid also have an option to highlight raid members who have aggro, so you have have a big red glow on whoever has aggro.