WP Captcha-Free is a lightweight plugin that blocks automated comment spam without using captcha (image verfication).
WP Captcha-Free generates a hash (aka token) based on several parameters like time (with a cushion of about 1 hour), post id, IP address, and browser user-agent which should not change between requests. 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.
That is not a proper comparison. Akismet blocks comment spam by analyzing its content while WP Captcha-Free does so by checking how it was submitted. I would suggest using WP Captcha-Free in conjunction with Akismet for best results.
WP Captcha-Free does not filter pingbacks and trackbacks.
Let me know, I will try if its useful enough and I have time for it.