If you are looking for a way of reducing comment spam, you may want to check out the Prove You Are Human WordPress plugin. The free plugin adds an image captcha box to your comment area to stop robots from spamming you.
The form shows nine pictures. The user has to show two pictures that are related in some way. It’s usually quite obvious what pictures are related i.e. two dog or two cat pictures. The user can also make the images larger by zooming using the form magnifying glass.
Once installed, the plugin also adds the image captcha form to your registration page. In the settings page you can set it so that only the image form is only added to the registration page. Strangely, you can’t do the reverse and only add the form to your comment area.
I wouldn’t recommend installing this plugin if you are not having a problem with spam. If are noticing a lot of spam comments or fake registrations from robots, it could be worth trying it out and seeing if you see a reduction in spam. Bear in mind that any anti-spam solution like this can discourage some of your readers from leaving comments (particularly if they get the captcha test wrong). This is a small price to pay if the amount of spam you receive is greatly reduced.