Some useful addons I've found recently while leveling my Mage:
Lightheaded is a very simple addon that displays quest information and comments from
http://www.wowhead.com in game, eliminating the need to Alt-Tab when you get stuck on quest.
Shield Monitor basically does as the name suggests, monitor how much more damage can a shield take or its duration before it fades. Useful for Priests and Mages.
Dance Music is an addon that plays a music when you do the /dance emote, the songs played are based on race and gender and also can be customized and must be added in mp3 format into the addon folder. No songs are added by downloading the addon due to copyrights issues. The readme also includes music suggestions and how to edit songs to loop.
Afflicted is an enemy spell timer mod for PvP (mainly arenas). This is for people who just want useful information, such as an interrupt is on cool down and you're safe to cast a heal or that Blessing of Freedom is going to be back up for an enemy Paladin soon. If you want to know the cool down of an obscure ability say Whirlwind then this is not the mod for you. New timers for abilities that are not included by default can easily be added under the configuration. The system is flexible enough that any enemy timer can be added including traps or totems.
