Best way to register a new Drupal user via a 3rd party application
I may be biased b/c I am the Services module maintainer, but, You don't want to be directly inserting things into the database.

You could REALLY break Drupal by doing this.

If you didn't want to use services module(which why wouldn't you?) you should really use Drupal's apis to put things in the database, like, node_save, or drupal_form_submit(which is what services does).

This ensures that Drupal actually enters the data and will lead to a cleaner overall integration with less problems.

In Drupal 6.x this json actually creates a user when POSTed to SERVER/ENDPOINT/user.

{"uid":"1","name":"admin","pass":"1a1dc91c907325c69271ddf0c944bc72","mail":"kyle@workhabit.com","mode":"0","sort":"0","threshold":"0","theme":"","signature":"","signature_format":"0","created":"1286571725","access":"1294792121","login":"1293782855","status":"1","timezone":null,"language":"","picture":"","init":"kyle@workhabit.com","data":"a:0:{}","roles":{"2":"authenticated user"}}