On the Venture Skills Blog, there's a new post that looks at one of the more recent advancements in keeping the spammers away from your blogs comments (one among many other uses) - re-captchas.

Commonly CAPTCHA is a visual image where the user is asked to type the word they see (or hear) however some provide a logic puzzle [...] Re-captcha works by asking the user for two words instead of the normal one, one word is known to the system and is the actual CAPTCHA the second is an unknown word, if the user gets the CAPTCHA then the users "guess" for the unknown word is recorded, over time a word is given a probability score and when high enough becomes a known word.

They Also include links to re-captcha modules for two of the popular CMS/blogging tools - WordPress and Drupal - to make integration easy.