Here is a list of addon's that I either personally use or suggest usage for other classes:

Essentials: all of these are background programs, none will directly effect appearance or functionality but rather extend most other addons for the better.

LibGui
LibSlash
LibSurveyor

Optionals: these all improve things but some require some setup.
BlackBox - this removes that giant respawn window replaces ti with a little bar. ::functions without needed activation
Dammaz Kron - keeps a running pvp deathblow chart, shows you all the numbers you care about, and all of the people that kill you as well as running totals of all that you have killed.::functions without needing activation
Phantom - Allows you to pick and chooses the pieces of the default GUI to make them invisible. type "/phantom" to use.
Pure - give a percentage breakdown on everything you can interact with very useful but takes a lot of tweaking to get it how you want it. "/pure" to activate the editor.
Squared - a different way to view your party members great for healers.
TomeTitan - for all of you that wish to whore this game for everything it has this will be your guide to it.
Waaaghbar Shockbar - a heads up display of tons of different customizable options of those listed is the hardest to set up and gain functionality, but in my opinion is worth it once you get it.
Twister - for chosen and Kotbs shows a friendly application to tell when your auras are active/inactive/expiring its only active when the correct character type is in use but essential to get full advantage of your aura twisting.

I'll continue to add more to this thread of new addons I test out, if you want me to try one out and see if its worth it or another one out there is better than one listed please post it.