patrys

This plugin will add a copyright footer to your feeds. Each footer contains a permalink, author info and the license.
Currently footers are not appended to the excerpts as it’s not common to steal these.

RSS License

Allows one to include the wpLicense copyright in blog feeds.

Jarkko Laine

WP License Manager is an easy to use plugin that will turn your WordPress site into a license manager that can be used for WordPress themes, plugins and other downloadable products.
Downloadable files are stored in Amazon Simple Storage Service (S3) to keep them safe from people without a license.
Possible uses for WP License Manager:
<ul>
<li>Hosting your own premium plugins and themes and serving updates to them.</li>
<li>Hosting private plugins that you don’t want to share in the open.</li>
<li>If you get more creative, the license manager plugin can be extended to be used for all kinds of digital goods; downloadable games, for example.</li>
</ul>

WP License Manager

Turn your WordPress site into a software license manager for WordPress plugins, themes, and other downloadable products.

RSS License vs WP License Manager