Id like to suggest just a few that would help you get started.
Fubar_RecZone (recommended zones for your level, and also gives you the walk paths)
Fubar (to put RecZone on)
AckisRecipeList (helps you keep track of your recipes, and lets you know if you're missing any and where you can obtain them. Also sits on Fubar.)
Carbonite Quest (All of your Questing needs. I cannot say enough about Carbonite!)
LightHeaded (This works extremely well in conjunction with Carbonite.)
The following are not addons, but they are extremely useful sites.
LootTables is a wonderful site when you need information about any of the professions. I normally always have this site open in my browser while I play.
Allakhazam is a great place to view all of your class spells, plus a wealth of other useful information.
WoW Insider is the best, imho, for keeping track of WoW community news. I get this in my news reader.
Hope some of those are helpful and useful to you!
