This Google Map mashup will help you find the zip code for anywhere in the United States. Clicking on the map or keying a zip code or address into the search box will zoom the map to the area of that particular zip code.
Links in the sidebar will zoom the map to the chosen state. New links will then appear to counties which in turn, when clicked, will load links to cities and towns in those counties. The map display includes a boundary of the state, county or city as a whole image overlay with a custom tile layer of zip codes.