Resources: If you're looking for quest, NPC or item information, chances are you'll find it here. Another good info site for many aspects of the game. 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. 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.