WordPress.org

Isn't this already a widget?

Yes, it is, in the older widgets plugin releases. For some reason, the "Authors" widget was removed for WordPress-2.2.

Can the display be customized in some way?

There are many requests for:

• Hide an author.
• Sort them alphabetically/by-registration/by-posts/whatever.
• Show only editors/authors/whatever.
• Make list collapsible.

The wp-authors plugin simply calls wp_list_authors in wp-includes/author-template.php. The lack of all of the above feature requests are due to the sparseness of that API. Implementing any of the above features will require either hacking the WP core or directly embedding SQL queries into the plugin. Both strategies subject the plugin to breakage with each WordPress release.