Securimage-WP adds powerful CAPTCHA protection to comment forms on posts and pages to help prevent comment spam from getting onto your site. CAPTCHA
This is a great plug. It works perfect. And the support by the author is absolutely the best I've ever had. Thank you, Drew.
I have this plugin also running on my non-Wordpress site: It just works (how nice is that for a change) and can be tweaked to your liking.
Hopefully its functionality will also work (preventing me being flooded with bots filling out every form I have) but I will know that for sure in a few days...
Very good plugin - after a little customization : )