My company bases just about EVERYTHING on county, it's the basis of our business. That said, we've got a TON of databases but the one that springs to mind for me is
http://www.zip-codes.com/zip-code-database.asp
Unfortunately, it's almost $140 to get the "sort-by-county" feature. It does, however, come with updates for a year.
I know there are companies that sell much more specific databases that this one - such as all liquor stores sorted by zip code. :P