sajidhussain

Instead of asking for both an email and a username on registration, this plugin lets you ask for email only, and uses that as username as well. Since the password is automatically generated and emailed to the user, this allows for sign-up form with a single form field: email.
For users that registered with different username and email address before this plugin was installed, their username and email address will be interchangeable on login.
The plugin offers a shortcode, [NP-registration], to add the registration form to a post or page.
Note: This plugin has not been tested with WP versions older than to 3.1.5. It should work with some older versions, but we can not guarantee it. However, if you are running an older version of WordPress, there are some very compelling arguments why you should consider upgrading. If you need help, there is plenty of free help to be found on WordPress.org, and Next Page, the authors of this plugin, offer a paid WordPress maintenance service.

Login Tweaks by Next Page

Use email as username to allow for single-field signup. Adds shortcode for adding registration form to post.

Login Tweaks by Next Page vs Participants Database