The Association of British Counties makes a number of resources available to aid in using the historic counties, including the following:
County Border Data in KMZ and SHAPE, eBook and Data Extract
on the Gazetteer of British Place Names
Match counties to Post Towns for your address database or look-up
on the Historic Counties Postal Directory
You may also like to visit the following to find out more about the historic counties:
Visit Wikishire, an encyclopaedia for the counties
The Historic
Counties Trust
a Registered Charity which aims to promote public awareness of the historic counties