WitchHunt is a Spell Alert addon.
It will warn you when someone is casting a spell or gains an ability.
Example: Ammo begins to cast Powers of Death
WitchHunt has gained several new features in 2.4 including filtering options on a per spell/ability basis and coloring options for the messages.
You can configure WitchHunt through: Esc -> Interface Options -> AddOns
Or open the configuration with /wh or /witchhunt
Example of color config: