I think it is important to know what you mean by "distance calculator".
When I read your first post, I thought you were meaning some sort of
distance calculator that can convert between metrics (i.e. take miles and
get kilometers). That seems to be what "Mike" thought you meant, as well.
If you are talking about a MapQuest / MapPoint style distance calculator,
then you are talking about some SERIOUS work.
Give us some more details and maybe we can point you in the right
"direction"...(pun intended )
You might lookup different zipccode databases. They one I just purchased
for $13 has all the US cities with Latitude and Longitude that you can
convert.