Just came upon the link to this website. While not foolproof, it may be helpful to some who have only a partial city or state name and are looking to identify the location:
http://www4.brinkster.com/kcscanning/poform.htmlIt works fairly well, however, the database seems limited to current ZIP codes.
In my area of the country (Massachusetts), for example, there are a number of newly defined ZIP codes within recent years (i.e. certain 021 ZIP codes have been changed to 024's as in Waltham, MA 02154 is now 02454 and Needham, MA 02192 is now 02492). Postmarks with the older ZIP Code information is not found on the above web site if the ZIP Code field is searched.
While it may not be perfect, it's still a good resource to know about ... and it's free!