Yes, I know, I'm a bit slow and behind the times, but I have now implemented recaptcha on my website for comments to blog posts. I don't do it to overly burden you, my loyal readers, but I do like the cause and the technological approach to helping solve it.
It short, and to plagiarise directly, "reCAPTCHA asks commenters to read two words from a book. One of these words proves that they are a human, not a computer. The other word is a word that a computer couldn't read. Because the user is known to be a human, the reading of that word is probably correct. So you don't get comment spam, and the world gets books digitized. Everybody wins! For details, visit the reCAPTCHA website".
So, commenting on this blog is now good for the world - so get commenting :-)