Pictcha is a very unique & secure CAPTCHA.
Note 1: If you have WordPress 2.7 or above you can simply go to 'Plugins' > 'Add New' in the WordPress admin and search for "Pictcha" and install it from there.
Note 2: If your WordPress theme doesn't have the 'comment_form' hook (i.e. Pictcha won't show up with the comment form), enter the following code right before the closing '' tag in the 'comments.php' file of the theme.
<?php do_action('comment_form', $post->ID); ?>