We've been getting a lot of comment spam on this site recently. All comments require approval and I've been using my handy-dandy (and recently updated) Comment Mail module to get alerted to new comments and quickly approve/delete comments. But with 20 spams a day, it's getting tiring to delete them all.
Enter CAPTCHA (which stands for completely automated public Turing test to tell computers and humans apart). Drupal's CAPTCHA module adds a very simple math test to the comment form to add another level of complexity to thwart spam-bots.
We'll see how this goes.