Akismet
Get an Akismet code from wordpress.com and enable Akismet which should be installed by default on your WordPress blog. If you don’t have it, you can get it here http://akismet.com/ .
captcha-free
Is a very easy to use plugin, all I have to do is install it on WordPress, and activate it. I haven’t gotten a single automated comment spam since installing it on this blog. I use to get 1-4 automated comment spam a day. now I have zero.
Best of all, users do not have to type in a captcha code to post comments.
You can also go into your discussion settings f0or wordpress under Settings Disscussions to reduce pingback and trackback spam.
