http://wordpress.org/plugins/hc-custom-wp-admin-url/
*** Please use any words other than wp-admin , admin or administrator , for better protection against automated attacks on wordpress admin login page.
*** Please use a very strong admin password and do not use the admin username as admin or administrator, use custom administrator user names of your choice while installing wordpress
WordPress admin username changer
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http://wordpress.org/support/view/plugin-reviews/admin-username-changer
http://wordpress.org/plugins/admin-renamer-extended/
This is for blogs that allow other users to register to their blog:
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http://wordpress.org/plugins/restrict-usernames/
*** Please note that before installing/using any plugin, please make sure that you take a backup of the wordpress database as well as the web pages/wordpress files.
*** Before installing a plugin, please check if the plugin is compatible with your wordpress version.