microkid

This plugin gives you full control over the permalinks for author pages. When using “pretty permalinks”, by default URLs to author pages are structured:
http://www.example.com/author/author-name/
With this plugin you can change the “author” part of these URLs into anything you like, for example:
http://www.example.com/editorial/author-name/ 
http://www.example.com/crew/author-name/ 
http://www.example.com/members/author-name/ 
http://www.example.com/posts-by/author-name/
The plugin will also automatically change links throughout your site that point to author pages. The URLs of author feeds are also changed.
Visit the <a href="http://www.microkid.net/wordpress/author-slug/" title="Wordpress Plugin: Author Slug" rel="nofollow ugc">plugin’s homepage</a> if you have any comments questions.

Custom Author URL

Customize the author slug in the permalink for author pages.

Konstantin Obenland

This plugin replaces the author slug with a sanitized version of the user’s display name.
This plugin will not prevent hackers from finding out your login name but the login name won’t be visible 
in the author archive’s URL, which works towards your friendly URLs with using your display name.
DO NOT use this on a site with more than 1000 registered users, as updating all of their nicenames at once might break your site.

WP Author Slug

Easy way to add some obscurity to login guessing.

But please update as it's over 2 years now.

Excellent

This plugin would probably work OK if you have a small number of users (e.g. if your site doesn't allow public login). But if you have a large number of users (e.g. if you are running ecommerce or allow users to register for other reasons), this plugin will probably take down your site.

The reason is that there are hooks that run when you activate or delete the plugin. It runs database fetch and update queries on every user in your database, all in-line on the request (rather than background via a cron). So if you have a thousand or more users it attempts to do a thousand or more database writes when you click Activate or Delete from the plugins list. This will likely grind your website to a halt. 

It would be better/useable if you could either choose to only apply these scripts to specific user types (e.g. Author) or if you could run the update in batches in the background.

Don't use if you have a large user database

Add a layer of security and prevent your login name from being shown in the author archive's URL.

Custom Author URL vs WP Author Slug