The Completely Automated Turing Test To Tell Computers and Humans Apart method of comment-spam prevention is, at best, a tolerable annoyance. It’s better than requiring registration just to leave a quickie note; on the other hand, lots of people can’t bear even so minor a speed-bump in the feedback process.

But if you’re going to muck up your website thusly, you might as well reap some greater benefit from it. reCAPTCHA, a second-wave filter guard, multitasks the retyping task by tossing in some digital decryption work that stumps computers.

The key to this new method is that it presents not one, but two words:

One of these is the real security word: Type this one correctly and you’re in. The other image is something that has mystified the digitizing software.

If people recognize that word, they type it in. This image will actually be shown to several people. If they all agree on what the word is, it will be considered accurately transcribed. And [CAPTCHA inventor Luis] von Ahn says it will be incorporated into the digitized copy of the book or the newspaper that it came from.

“And the number of words that we’ve been able to digitize like this is insanely large, it’s like over a billion. It’s like 1.3 billion by now,” von Ahn says.

In the journal Science, he and his colleagues report that over the last year Web users have transcribed enough text to fill up more than 17,600 books, with better than 99 percent accuracy.

So we have spambots to indirectly thank for digitized archives of the New York Times and public-domain books. There’s a certain karmic balance coming out of that.