Implementing location proximity search (for Belgium) with Drupal and Google Maps
This tutorial will show you how create a Google Map with location pinpoints in Drupal 6, and how to let your users filter the map based on postalcode.
This walkthrough was written to create a map for Belgium, as I encountered some gotchas while doing so. However most of it will apply to other countries as well, and you might even skip some steps that are only required for the Belgium use case.