Resources:
http://www.thottbot.com/		If you're looking for quest, NPC or item information, chances are you'll find it here.
http://www.goblinworkshop.com/		Another good info site for many aspects of the game.
http://www.classesofwarcraft.com/		Nice talent planner as well as other useful class information.
User Interface / AddOns:
Curse Gaming		Lots of different AddOns are hosted here. Has a nice latest changes list and allows you to add your addons to your favorites for easier monitoring.
ui.worldofwar.net		Similar to Curse Gaming, but they don't have a favorites list for your addons. Still a nice addition :)
WoW Guru - UI Mods		Another site were you can find many different AddOns.
Auctioneer		Provides your item tooltips with information on the average auction prices, number of times seen at auction and estimated vendor buy/sell price.
Enchantrix		Provide your item tooltips with information on the elements that a particular item will disenchant into (uses LootLink, included).
Gatherer		Track their herb, ore and treasure gathering activities on your world and minimap.
Miron's UI Mods		BuffStatus, SunderThis, Logger and other AddOns.
CT_RaidAssist		Commonly used AddOn to make life in raid groups easier. Recommended, especially as tank or healing class.