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.

mattdu

This plugin uses the custom taxonomy features of WordPress to create “bylines” — essentially, tags representing authors who contributed to a post. The differentiating feature of this plugin is that it should not require any modifications to your theme files (assuming it is already displaying the post author).
This plugin would be useful for publishing situations where the majority of your content is from contributors who you don’t necessarily want to have access to your back-end. It still conveys the appearance that the authors are members of the site (it provides a built-in archive page for each Byline). If your theme displays tag descriptions, you could use the Byline description field to identify guests vs. “staff” writers or provide other author information.
You can see an example of this plugin at work at http://thedailycougar.com

Byline

The Byline plugin is simple and suitable for multi-author WordPress sites.

Best For Multi Author

Solves the co/multi-author problem without modifying the theme. Uses a custom taxonomy, "Byline," that replaces the Display Author.

WT Co-authors vs Byline