There are several free directories of area codes divided by state. A user-friendly interface is located at allareacodes.com, which provides a list of all the area codes for each state.
In order to be able to determine where in the state each area code is located, users can visit nanpa.com. This website allows the user to click on each state and pull up a map of the state with the boundaries for each area code shown. Though the map is not precise, it does provide guidance to determine where the area code is from for larger states like California or Texas.