Originally Posted by sunsmoon

Shadow Priests had/have a mod similar to this (during BC) called Face Melter - Face Melter - Addons - Curse

You had a menu where you could set up each skills priority and disable some (ie: disable shadow word death, disable below % health, etc). It would show a row of 3 icons. the leftmost icon was your last cast skill, middle was the one you should use next (when it lights green!), and right was the next one in the priority queue.

It is viewed as a crutch by many people, but it's great for teaching new shadow priests how to prioritize spells instead of locking into a pseudo cast sequence. It's also helpful on fights where you might be so distracted by other things that monitoring debuffs and cooldowns can detract from your main role.