wowwiki - An extremely helpful site for both new and experienced players. Advice on professions and talent builds, loads of WoW lore, gameplay guides, and more. Carpe Orcium now has its own profile page on wowwiki, found here!
Macro guide - Technically a subsection of wowwiki but helpful enough in its own right to warrant it's own listing, imo.
Thottbot - The World of Warcraft searchable database. Contains info on mobs, items, quest & questgivers, recipes, pretty much you-name-it. Note that thottbot obtains much of its information through the thottbot addon so installing and running it while you play can be helpful in keeping this awesome site going. You can download this addon either with the Cosmos UI package or by going here.
The Armory - Whether you're curious about what talent build your guildies (or others) have or what gear another player used to pwn you in the BG, this site will allow you to search for them and view various vital stats. Can also be used, roundaobutly, to see if a name you've got your heart set on is available for a future alt.
WarcraftRealms - This site is a WoW census site, breaking down the percentages of each character race/class combo on WoW, either as a whole or broken down by server, faction, guild, etc. This site functions by gathering its information through the CensusPlus add-on so installing and running this addon can be helpful in keeping this site's information accurate. You can download this addon either with the Cosmos UI package or by going here.
Petopia - Mostly just helpful for hunters, this site details a number of useful facts about the games tameable beasts. Has some pretty pictures, though. :)
Shadowpanther - Rogue site, mostly about gear but some advice on talent builds and high-level questing.
Urban Dictionary - Lots of WoWers use expressions like w00t, l33t, pwn, n00b, roxxors, ftw, roflmao, lawl, and a bunch of others or maybe even stuff like |O4\\/\\/|\|3|) or |>|-|34|2 . This site will (hopefully) tell you what the heck they're talking about. ¯\(o_º)/¯