Anonymous Checkout & User Registration
Modules that can be used to customize the registration/login process during checkout.
Before completing checkout
Redirect to login page before checking out
If you want to send users over to a customized registration or login page, Commerce Checkout Redirect will do that when they click on the checkout link.
This module redirects anonymous users to a page where they can login or create a new account when they try to checkout.
Add a customer login step to checkout
The Commerce Login Step module adds an extra page to the checkout process along with pane that allows a customer to log in or proceed without logging in.
Two other modules that appear to do something similar are:
Commerce Extra includes two panes and a login page. One pane is for unregistered users and one for logged-in users. Still under development.
This module extends that pane to provide e-mail validation and an inline login form via AJAX. If the e-mail address an anonymous user enters is already associated with a user account, the checkout form will expand with an optional password textfield and login button for immediate login.
After completing checkout
This module creates a new pane which (defaulting to the Complete pane) that allows a customer to complete their registration. Great for using with anonymous checkout to give customer a chance to create the account after checking out.