Okay, so I got tired of dying to random trash mobs and considered what I could do about it, and I thought "the first thing I need to know is when I get aggro", so I made a mod that tells me:
It has options for sound, message, LowHealthFrame flashing, and comes prepacked with a female voice that says "Aggro" when that is the case (from Squishy). It defaults to printing a red warning to your Big Wigs message frame, but also supports SCT5, MSBT, Blizzards FCT and the UIErrorsFrame, where it displays "Aggro from Anubisath Guardian!", for example.
It can also notify you when you lose aggro, if you're a tank, for example. It has an option to disable when you are playing solo.
BanzaiAlert was made as a proof-of-concept implementation for LibBanzai-2.0. I've used it for a few raids now, and I don't think I can live without it any more. The way it works is just scanning your raid for targets and then firing events when the target status changes.
If you want to request a feature or report a bug, please visit the wowace.com ticket system for BanzaiAlert.