I finally got fed up with manually monitoring the spam messages, but I didn't want to turn off anonymous comments. I figured captcha was the answer, but what was the easiest way?
photo.net, the awesome photographry site built and run by Philip Greenspun is built on a similar platform to this site, and I knew it had captcha. Philip is true to his word on the importance of open source software, and the api and code of his site is publicly available. I discovered that photo.net uses a free service called reCAPTCHA - and it's brilliant! It gives you two words to type in. One that it knows the answer to (to check you're a human) and the other is a curly word that the OCR system at Carnegie Mellon University couldn't figure out while scanning books for the public good. I assume they run the unknown word through a number of captcha queries and pick the most popular interpretation.
Now that's really crowd-sourcing!
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