These two addons work in conjunction with each other. Tidy plates gives you customizeable health bars above the heads of enemies. They can also be configured to include casts and even show cast bars that indicate whether the spell can be interrupted or not. Threat Plates are great for all roles, but are amazing for tanking. The colors can be customized, but by default the bars are red if you have aggro, yellow if you are losing aggro, and green if you do not have aggro. These are in sync with wow's aggro system, so they seem to be spot on. Another feature is turning specific mobs plates a certain color. Having trouble finding omnitron's oozes? Make their specific plates neon orange and size 36 font - you wont ever let one slip past.
http://wow.curse.com/downloads/wow-addons/details/tidy-plates.aspx
http://wow.curse.com/downloads/wow-addons/details/tidy-plates-threat-plates.aspx
