This thread is so I no longer have to put websites into /g, this list will probably grow as i remember more stuff.
mods:
www.wowace.com - go there look, learn and read. Most information is in the forums
www.curse.com/downloads/addons - where the majority of mods are released ie. stable.
world info:
www.thottbot.com - has pretty much every in game item, npc, trade skill, recipe everything. My favorite for the rabid fanboism and uninformed comment. This site is funnier than the glass house, like the Einstein factor meets Australian idol.
www.allakhazam.com - same as above, crapper UI, adds and a continual "join up for allashacrap gold for only $89999". I hate this place.
www.wowhead.com - as above, probable has the most informed comments.
www.wowwiki.com - as above but as a wiki, has some really good info, like reading thottbot but without the crap.
indepth class info and theorycrafting:
www.elitistjerk.com - the only wow forums worth reading if u really want to learn and understand your class. Has many guides on all classes and is pretty much regarded as "cannon".
www.tankspot.com - like elitest jerk but focused on tanks, very good.
www.maxdps.com - dps calculator for most dps classes as well as EXTREMELY USEFUL GEAR RANKINGS. If u want an upgrade look here.
raid guides:
www.bosskillers.com - as it says, how 2 kill shit.
wow news:
www.worldofraids.com - up 2 date info on the wowverse, has only relevant content. If a big patch is coming they'll know, class changes its here.
www.wowinsider.com - as above but with more fanboism and crap, still useful.
www.wowradio.com - wow and other gaming podcasts, has some interesting shows, check ocktail and hordak or the dungeon master. Also has interviews with method after their vashj world first, nihilum after there illidan world first etc.
edit: typos.