Links for WOW that you may find useful. Feel free to add others I may have forgotten in the comments section. Give the link and information on it. I've tagged this under WOW in case you ever need to find it in my tags.
http://www.wowwiki.com - Everyone knows this one but it's good info. Especially in say, instances where a boss may require a strategy. The other day our team wiped twice until they LISTENED to the strategy I was giving them from this website LOL Plenty of other information as well since it's a wiki.
http://www.thottbot.com or
http://www.wowhead.com - Common knowledge also. WOW database and comments on where to find things, people, quests, places, vendors, trainers or drops. You can also download an add on and contribute.
http://www.wow-loot.com/ Kaliban's class loot lists - Easy to navigate gear list by CLASS and area where it's dropped. Yes there are other sites but they often list it in an ugly way when you just want to know what YOUR dude needs and where, darnit.
http://www.wowarmory.com/ - Spy on your friends or other people (what armor are they wearing!)
http://www.wowace.com - click on the Ace Updater and download it for quick one click upgrades of Ace based Add ons. Listing of probably over 100 different Ace add ons you can add with one click as well. Or if you prefer to manually download zips, just go to their addon listing.
http://www.wow-pro.com - Leveling guides for10 bracket levels, up to 51-60. No outlands stuff sorry. But if you follow the guide, you'll understand power leveling and be able to do it on your own. It truly does speed up leveling to an amazing degree and efficiency. Especially if you have an alt that's rested.
http://wow.crafterstome.com/home.html - Crafters Tomb - recipe lists and crafting guides for your 2 professions
http://auctioneeraddon.com/dl/AuctioneerOnly/ - Auctioneer. Don't leave home without it.
http://petopia.brashendeavors.net/ - Petopia. For you hunters looking for the right pet for the right place (PVP/Instances/Grinding)
http://www.warcraftrealms.com - Cenus and balance information on your realm. Will even tell you about your guild, it's breakdown on characters and levels and what rank it has (top 50 or better). You can download a census program and contribute manually to see more accurate stats. I did, it's pretty cool! Our guild went from rank 48 on the server to rank 22 thanks to recruiting AND the fact I kept taking census' or we'd have never known.
http://wowhunters.coldfront.net/forums/ - Hunter forums. I bet if you did an altavista search you'd find these for other classes.
http://armory.mmo-champion.com/create_sig.php - Create your awesome WOW banner sig that changes on it's own as you level. Useful for boards or your LJ profile.