I use cleantalk on my Drupal website (but it's also available on other systems, such as WordPress) and only get a few spam comments a month. Its also cheap: 12

I use cleantalk on my Drupal website (but it's also available on other systems, such as WordPress) and only get a few spam comments a month. Its also cheap: 12 dollars a year and you can earn extra credits reporting spam comments that slip through.