Useful websites and why

Class Mechanics and Rotations:

www.elitistjerks.com A forum-based website with an excellent Class Mechanics section broken down by class. There's a lot of information and slush to separate there, but it's a fantastic place to learn about how specific classes work and the various specs available for raiding. (They also have a less-helpful PvP section.)

Gear Checking:

be.imba.hu Not a perfect website, but it' good for approximating on where gear upgrades will be coming from as well as an approximation of where you could be raiding. The best aspect of this site (which picks your character up from the armory) is that it will give you advice on gemming and enchanting.

www.maxdps.com Again, not a perfect website in terms of what they're trying to tell you. This sight is mainly used for getting a general idea of what gear might be better. You can enter your toon name and realm and it will pick it up from the armory. Don't pay attention to any of the 'dps' statistics since it's false information (the rotations are wrong and there are too few fights that are stationary).

http://www.themoonkinrepository.com/index.php

Moonkin bible.