I've modified the comments system to use the reCAPTCHA system, rather than my home-made one which relied on the user to count the number of animals in a photo.

reCAPTCHA helps digitise old books by letting people type in words which are a challenge to OCR systems. More information can be found by clicking on the link above.

UPDATE:

Not impressed so far. Only an hour after installing the reCAPTCHA, I received 2 spam comments. Under my old system I rarely got any spam comments. I'll give it a day and if it stays like this I'll put the old CAPTCHA back.

UPDATE UPDATE:

Two hours later and another spam comment. I'm putting the old CAPTCHA back. I had hoped that reCAPTCHA would work. I didn't expect to get more spam with it installed.