The North American Numbering Plan Association website allows people to click on a state within the United States and then view all the area codes within that state. Larger states like California have inset areas to click on for cities with multiple area codes.
Maps of World's website tries to actually put all the area codes onto a single map, but cannot seem to fit them all and winds up missing a few as a result. Arizona's 602 code is one example. Just like the NANPA site, though, when a single state is clicked, all the area codes for that state show.