List of US counties ?
 

I know this is an old post but for those of you who may need this data,
I found a great website for inexpensive developer and marketing lists
for zip codes. Plus they give free updates.

http://www.listfactor.com

They have lists for united states zip codes, cities, and counties. I
needed the cities and found that there are ove 30000 in the united
states! Didn't know that.

Brandon