The Pro Blogger version of Akismet is a paid product. The Standard version is available for personal use at no charge for all WordPress® customers (for activation key go to http://akismet.com/personal/).
Akismet is a WordPress® spam filter that monitors your site's new comments, trackbacks and pingbacks, running hundreds of tests to block spam. Pre-installed in your WordPress site, Akismet simply requires you to enter the provided activation key to get started.
With Anti-Spam powered by Akismet, you get:
Why do I need the Pro Blogger version of Akismet?
Akismet is free for personal use. If you have a professional blog that generates more than $500 per month, Akismet requires that you use the Pro Blogger version.
Learn More About Akismet (Pro Blogger Version)
If you want to activate the free version of Akismet on your WordPress site, visit http://akismet.com/personal/ for a personal use API key.