Since I have upgraded my site to Drupal 5 I have added to Captcha to get rid of the bloody spam comments. Originally I was going to just use the math or other basic captcha's to implement a very clean captcha.
But I was listening to Security Now, Leo and Steve were talking about reCaptcha which not only a allows the use of captcha, but also helps with other projects. Here is a quick snippet of what these projects are
To archive human knowledge and to make information more accessible to the world, multiple projects are currently digitizing physical books that were written before the computer age. The book pages are being photographically scanned, and then, to make them searchable, transformed into text using "Optical Character Recognition" (OCR). The transformation into text is useful because scanning a book produces images, which are difficult to store on small devices, expensive to download, and cannot be searched. The problem is that OCR is not perfect.
So what they do is take these mis-converted words and use them as the captcha image and people responses are passed back to the projects to allow the projects to quickly fix up all the missing words that the OCR's couldn't get.
So if you do need to have captcha on your site, you might as well make it help other projects at the same time. So far for me it is working great. If you want to use recaptcha then you will find it at http://drupal.org/project/recaptcha