News:

MMO-Champion - Great place for up-to-date news and unofficial forums!

Forums:

Battle.net [EU] | [US] - Take a look at the official forums, but beware of trolls!

Quests:

Wowhead - Awesome site for both quest- and item-lookups!

Professions:

WoW-professions - One of webs best profession guides!

Add-Ons:

Curse - Gives you addons for all purposes!
WoWInterface - Another great source for all your addon needs!

All-around useful AddOns:

Auctioneer - Keep yourself updated with the current auction house prices.
Bartender4 - Addon to customize your action bars.

Common PvE addons:

Deadly Boss Mods - A must have for raids in many guilds!
Recount - Damage meters, pew pew.
Omen Threat Meter - Threat meters so you know when you'll pull aggro!

Common PvP addons:

Gladius - One of the best arena helpers out there.
Capping - With this you'll always know all battleground and arena timers!
Spell Alerter - Always be aware of all important spells being cast, highly customizable.