Some addons worth thinking of:

First, Curse client, wich is a program to install Addons for games!

Good ones

Questhelper - If you want help with questing.. works fine :D
Cartographer Really good one if you want to see instances on the map and the level of the mobs of the Zones.. Req addons:

ace3
librock-1.0
libtourist-3.0
libBabble-zone-3.0


Got some more, i will post them later on.. its "nice ones" but not required..
Nice ones
Atlas - an addon wich you can see wich items a boss drops in an instance
Healbot - for holy pala or healers
Titan panel - Wich makes your UI be much much smaller!
Palainstantcast - FOR PALA ONLY! shows when your excorcism turns to instant cast!


some more ones I use
X-perl unitframes - makes the UI frame of party + raid look a little bit better
Bagnon to highlight in your backpack and shows the frames in the bag.. Nice one
Sell junk - to sell all the "junk" in your inventory.. also a nice one