More Useful Add-ons

BuffWatcher

http://www.wowinterface.com/downloads/info8168.html

Identifies when you lack a flask, or an aspect or any number of other effects which can be tuned to your liking. Will also identify when LnL has proc'd, giving you a countdown of the effect.

GTFO

http://wow.curse.com/downloads/wow-addons/details/gtfo.aspx

Provides a clear and identifiable warning if you're standing in something you shouldn't be. In many cases it is able to pre-warn you about an effect that may not yet be visible. Very useful in situations that require extensive amounts of situational awareness.

ViperNotify

http://wow.curse.com/downloads/wow-addons/details/vipernotify.aspx

Plays an audio alert any time Viper Aspect is active when you're within a set % of mana, and the aspect is still on. Useful for anyone who sometimes forgets to switch out. I generally leave this as 95%.

Power Auras Classic

http://www.wowinterface.com/downloads/info8579.html

A graphical add-on that will display a symbol or text under giver criteria. This is a somewhat complicated add-on but is unmatched in customization and application as an alerting add-on. An excellent, if extreme use of this add-on is exampled below, each colored piece can be set to a specific buff/debuf, cooldown etc. giving visual guides to a situation. Personally I have a hard time tolerating the add-on for common use, but in fights like Lich King when dealing with http://www.wowhead.com/spell=70337/necrotic-plague an easy to identify symbol can be used to alert you the moment you're stricken with it, so there's no delay for recognition.