I needed to figure out how to allow a user to use either their username or email address to log into WordPress. It took a bit of rooting around but the final piece of code ended up being pretty simple.

function login_with_email_address($username) {
	$user = get_user_by_email($username);
		$username = $user->user_login;
	return $username;

Place that in your functions.php file and you will be able to sign into your WordPress site using an email address or by default, the username.