On the NETTUTS.com website today there's a new tutorial showing how to create a simple signup form (username, password, email address) for your site with a MySQL backend database.

In this tutorial, we are going to be creating a user signup form that adds a user to a database, and then sends out a confirmation email that the user must click on before their account will be activated.

Everything's provided - the database structure, the code to make the submit process work, how to confirm the user once they come back in and creating a simple templating system for the emails it sends. You can download the full source to get it all in one bundle.