WP Captcha-Free is a lightweight plugin that blocks automated comment spam without using captcha (image verfication).
WP Captcha-Free blocks automated comment spam without resorting to CAPTCHAs. It does so by validating a hash based on time (and some other parameters) using AJAX when the form is posted. Comments posted via automated means will not have a hash or will have an expired hash and will be rejected. Unlike using a captcha, this does not place any burden on the commenter.
WP Captcha-Free generates a hash (aka token) based on several parameters like time (with a some cushion), post id, IP address, and browser user-agent which should not change between requests (within a short period of say a few seconds). When the comment form is posted the plugin uses ajax to get a hash value and adds it to a hidden field. On the server side it verifies if the hash is valid or not. It uses adds random salt to the hash so that it cannot be guessed.
Please let me know what you think about the plugin and any suggestions you may have. If you use the plugin please rate it. If it doesn't work for you do let me know so I can fix it.