This extension Easy Digital Downloads extension will override the original purchase button for every download product that has price of 0 (free), to a direct download link, bypassing normal shopping cart mechanism, but still maintaining awesome features of EDD, such as download logs report, and requiring user to register before download.
Tested up to Easy Digital Downloads 1.9.9
This extension was created by vafpress: http://profiles.wordpress.org/vafpress/
For more information, please visit the WordPress plugin directory for this extension: http://wordpress.org/plugins/easy-digital-downloads-free-download/