Working with numerous membership associations, we've been tasked with creating postcode searches on a number of occasions. The above example is WaterSafe. WaterSafe forms a national scheme of approved plumbers. The scheme is backed by 6 of the UK's largest water suppliers and the "Water Industry Approve Plumbing Scheme" (WIAPS).
The search system has around 6000 plumbers across the country. The map shows those nearest to you by converting your postcode to a longitude and latitude, then calculating the distance. The extra search criteria further filters down the results.
The integration with Google maps gives a clear indication where each plumber is. Clicking on their name in the list creates a pop-up including further information.