These are some websites that I frequent to obtain information on a plethora of subjects.
ElitistJerks.com – This one is especially for the number crunchers. Do you like theorycrafting? Discussions on BiS gear, min/maxxing stats? Then this is the site for you!
MMO-Champion.com – An abundance of information regarding almost anything … frequent updates of patch notes, who’s who in the world of raiding, and forums galore for all of your questions and research!
Tankspot.com – Excellent resource for boss strats and videos, among other things. Both in normal and HD qualities!
WoWHead.com – The main hub for all things related WoW; spells, talent calculators, maps, quest related queries, professions … you name it, they’ve got it!
WoWWiki.com – Kind of like the “wikipedia” of all things WoW related.