Comment Form – E-mail Not Required!

Settings > Discussion > Other comment settings > uncheck Comment author must fill out name and e-mail

Update

Well, as far as WP is concerned these are glued together. You can add your own check for empty name, something like this (not tested):

add_filter('preprocess_comment', 'require_comment_author');

function require_comment_author($commentdata) {

    if ('' == $commentdata['comment_author'])
        wp_die('Error: please enter a valid name.');

    return $commentdata;
}

But then will also need to modify form generation so name is marked up as required and email not…