These are some useful links I use almost daily when playing WoW.
Allakhazam to check quest details and spawn locations for certain mobs. Also to check the best grind areas for a primal or other item I'm not used to farming.
WoW Wiki to find out more complicated information which is harder to search for on Allakhazam, or may not be in the comments for the things I am looking at. I also use WoW Wiki for enchant and gem information.
Wowhead if the comments on Allakhazam aren't helpful or if the information looks a little strange. I tend to find there is less information on Wowhead, but what there is tends to be of a higher quality.
Almost everyone knows them, but I wanted to make a post with them in anyway.