For particular airports, www.airnav.com
has accurate freqs. Just go to the "Airport" section and type in the name or identifier. For Class D airports, it'll give you ATIS/ASOS/ ground/tower/clearance if they have it and the air route traffic control frequency in the area of the airport. For B and C airports, it will have ground/tower/clearance and the approach and departure frequencies for the airport. For example... http://www.airnav.com/airport/KIND
As for Air Route Traffic Control frequencies, there's a link on the main page( www.liveatc.net
) that will give you a site for those. They're not terribly accurate and not all the freqs are on there, but it definatly helps.