Actually, in providing this utility, we were mostly having fun.  We'd secured the data and created machinery, internally, to quickly connect city names with any zip code you might happen to throw out (an ability essential in creating your StreetList).  It occurred to us that some users might find it convenient to have this ability at hand, and its reverse: the ability to  throw out a city name and instantly see all zip codes applicable to it.  So, we made a simple application that does this for you.  Just click on its icon; operation is self-explanatory.  The one drawback: as mentioned elsewhere, in some cases the city name, that's connected to a zip code, is not very accurate.  Blame the Post Office for that.