Ask HN: Database of cities

I need to be able to validate a user's location. For this I was hoping there was a database of cities I could use. I don't mind paying for it.

Does anyone know of a good database of cities, states and zip codes? Only need Us but if world or even multiple countries were available that would be nice.

Another possibility that would be nice is if I could simply check the location against Google or Facebook but couldn't find any API for this. Maybe someone could point me in the right direction?