I had so many spam comments on my blog, so I had to install reCAPTCHA.
I hope it blocks some spammy comments.

Just in case, who doesn’t know what reCAPTCHA is go below link.

http://recaptcha.net/learnmore.html

I test so many reCAPTCHA plug-in and these are the best. I had to install 2 plug-in for more secure spam block.

And below link is the plug-in that I installed on my blog.
NotCaptcha

http://wordpress.org/extend/plugins/wp-notcaptcha/

Author Homepage » http://www.webjema.com/
Plugin Homepage » http://notcaptcha.webjema.com/
Download » http://downloads.wordpress.org/plugin/wp-notcaptcha.zip

WP-reCAPTCHA

http://wordpress.org/extend/plugins/wp-recaptcha/

Author Homepage » http://www.blaenkdenum.com/
Plugin Homepage » http://www.blaenkdenum.com/wp-recaptcha/
Download » http://downloads.wordpress.org/plugin/wp-recaptcha.2.9.6.zip