Login into WP with your email address instead of your username

functions.php

<?php
/** Plugin Name: (#90328) Login with E-Mail address */
function login_with_email_address( $username ) {
    $user = get_user_by( 'email', $username );
    if ( !empty( $user->user_login ) )
        $username = $user->user_login;
    return $username;
}
add_action( 'wp_authenticate','login_with_email_address' );

Source: http://wordpress.stackexchange.com/questions/90328/email-address-or-username-used-to-login-in-wordpress/90329#90329

Makes sense to pack it into a plugin. But you can slo add that to your functions.php file