Gargron

Displays authors of a collaboration post in place where only one is shown by default. Uses custom fields (key=<code>coauthor</code>;value=user’s username; one per user). Note: If it doesn’t automatically display the co-authors you must manually add <code>if(function_exists(wt_the_coauthors_link)): wt_the_coauthors_link(); endif;</code> into your theme (in PHP <code><?php ?></code> brackets, of course).

WT Co-authors

Using WordPress Custom Fields, and without editing any template files, lets you credit all authors on collaboration posts.

flocsy

Authors Widget shows the list or cloud of the authors, with the number of posts, link to RSS feed next to their name, avatar. It is useful in a multi-author blog, where you want to have the list in the sidemenu. 
The widget can also display an “Author Cloud” if <a href="https://wordpress.org/extend/plugins/seo-tag-cloud/" rel="ugc">SEO Tag Cloud plugin</a> is installed.

Authors Widget

Authors Widget shows the list or cloud of the authors in the sidemenu.

WT Co-authors vs Authors Widget