To use the plug-in, you need to purchase it -- for zero $$ -- and get a license key.
Having said that, it works great! I have tried GASP, Akismet (NOT free for most sites), and Captcha (which uses math problems), and Spam-Free WordPress has worked better for me than any of them. I monitor five sites, and am seeing fewer than one spam a week for all of them combined.
A five-star product, reduced by one star for the "purchase."