Useful Add-on Sites:

1. Curse.com
2. WowInterface

Add-ons:

1. Omen - THE threat meter. You MUST get this add-on. Period.
2. Deadly Boss Mods - Another MUST-get. This gives raid announcements, warnings, & boss timers.
3. Bagnon - open up all your bags in one window.
4. BankItems - view your bank items from anywhere in WoW (as well as everything in your alts' banks), without actually being in a bank.
5. Atlas - THE useful add-on for instances! Instance maps, what the bosses are, what their loot is, etc etc etc.
6. AtlasQuest - Adds instance quest info to Atlas
7. AtlasLoot Enhanced - a mod to Atlas that lets you search all the available boss loot
8. Suki MiniMapCoords- adds coordinates below the mini-map. Almost everyone in WoW uses something like this.
9. Outfitter - manages your outfits, allows for autoswitching of gear in certain situations, and hotkey your wardrobe changing.
10. RolePlaying Helper - want to yell "DIE DIE DIE!" when you're attacking a mob? Or spout random phrases at specific actions? This is the addon for you.
11. Bartender - allows you to completely customize your action bars.
12. Fubar - adds a control strip to your screen for your mods, instead of crowding buttons around the minimap.