Link to registration page in Wordpress

<?php echo wp_registration_url(); ?> 

https://codex.wordpress.org/Function_Reference/wp_registration_url

The following will return the registration url:

<?php

    echo site_url('/wp-login.php?action=register');

?>

UPDATE:

To get the registration url with a redirect to the current page use:

<?php

    echo site_url('/wp-login.php?action=register&redirect_to=' . get_permalink());

?>

Since 3.6, there is now a func: http://codex.wordpress.org/Function_Reference/wp_registration_url

<?php echo wp_registration_url(); ?>

You can override it with the register_url filter.

add_filter( 'register_url', 'custom_register_url' );
function custom_register_url( $register_url )
{
    $register_url = get_permalink( $register_page_id );
    return $register_url;
}

I know this is an old question, but for anyone picking it up use wp_register().

It automatically determines if you're logged in and supplies either a link to the admin section of the site, or a link to the register form.

It also respects the settings in Settings -> General -> Membership (Anyone Can Register?)