This WordPress plugin provides a widget to showcase your site’s authors. When configuring the widget you select the authors and choose whether to display them with their avatars and choose how many posts to display for authors.
Please download the plugin directly from the WordPress.org repository:
Featured Authors Widget
You configure the widget in the usual way in the wp-admin -> appearance -> widgets section:
Here’s how it looks in the default “TwentyEleven” theme:
Add a little CSS to your theme and here’s how it can look (As seen on the Article-3.com site):