Wowhead - Probably the best of the "new generation" of sites with item links, talent calculators, etc.

Allakhazam - An older reference, but still one of the most complete and frequently-visited.

WoW Loot - An invaluable guide to the gear available for your class and spec in the Burning Crusade zones and instances. Also includes raid drops cross-referenced by classes that would gain the most benefit from them. A must-read as you prepare to gear up for the level 70 raiding scene!

WoW Wiki - An incredibly comprehensive and user-editable encyclopedia of all things WoW. The first place most of us go to when researching a new boss or instance.

Bosskillers - High-quality (and often illustrated) boss guides, with in-depth advice for all roles.

SWotH on the Armory - The Official World of Warcraft Armory site - with SWotH already loaded up for ya.

WowJutsu - A web site listing guild rankings for our realm, Uther. Also shows the progression of each guild.

The SWotH Pack - A collection of recommended mods for SWotH members. Right now this isn't very relevant, but look for an update in the near future to include highly-recommended mods and in-game DKP and bidding aids.

Curse Gaming - A one-stop shop for the most popular mods.

WowAce - Home of mods built on the Ace platform, emphasizing efficiency and customizability. Check out Pitbull for one of the most customizable unit frame systems available!

CT Mod - Home of CT Mod and CT RaidAssist. These aren't nearly as critical as they once were, but they are still useful and widely-used.

Leeroy Jenkins - an UBRS run that led to infamy.

Onyxia Wipe - 50 DKP minus! A true classic.

Lord of the Rings in WoW-speak - 'Nuff said.

You Got Owned - substitute in the name of your favorite rival/raid boss/deposed foreign leader.

QueueDance - not as relevant now as it was in the good ol' days, but disturbingly addictive.

Paladin TPS Reports - soon-to-be legendary thread on the paladin spec dillemma. "So that means that when I make a mistake, I have eight different people yelling at me on Vent about it. That's my only real motivation is not to be hassled; that, and the fear of losing my raid slot. But you know, Bob, that will only make someone work just hard enough not to get /gkicked."

World of Roguecraft - a series of long but very well-made videos (with great music) about how overpowered rogues are. Also check out episodes Two and Three.

Knitemare's Cats - not really WoW related except in its relation to Barrens chat. Nevertheless, a guaranteed mood-improver.