Get the user location during registeration process
In Drupal 6, the obvious answer would be use the Location and Gmaps or OpenLayers module. I've used it on many many sites and it generally works well. Another very good alternative in Drupal 6 that I just found now is HTML5 user geolocation, that looks awesome and its what drupal.org uses!

However if I was using Drupal 7, I would definitely use the Geolocation Field module. I've tried it out and its really awesome and easy for users.