www.mollom.com

             The web is changing. User contribution is now what makes or breaks a site. Allowing users to react, participate and contribute while still keeping your site under control can be a huge challenge. Mollom is a web service that helps you identify content quality and, more importantly, helps you stop spam on your blog, social network or community website. When site moderation becomes easier, you have more time and energy to interact with your community. 

Top 5 benefits:

• Kills comment, contact form and registration form spam
• Overall content quality improved
• Burden of site moderation reduced
• Faster community growth
• It's FREE!

 Top 5 features:

• State of the art text filtering and content analysis
• Works with all languages
• Image and audio CAPTCHA service
• Detailed statistics
• High-availability infrastructure

Link: http://www.mollom.com/

www.akismet.com

       Akismet is a hosted web service that saves you time by automatically detecting comment and trackback spam. It's hosted on our servers, but we give you access to it through plugins and our API.
How does it work?

      Each time a new comment, trackback, or pingback is added to your site it's submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down. As a result, you don't have to waste your time sorting through and deleting spammy comments from your blog.

Link: http://www.akismet.com/