Luis helped create the CAPTCHA -- those squiggly letters you have to type into web forms to prove that you're really human -- and later, reCAPTCHA. As he explains in this video, once CAPTCHA went worldwide he realized that he helped waste a lot of people's time everyday by having to enter this info (even if it was for a good cause -- security) so he wondered if there was a way to make this time more useful.
The result was reCAPTCHA, which combines with the Project Gutenberg and Google Books to use reCAPTCHA to help decipher words that computers can't understand when scanning books into digital format. That's why reCAPTCHA has two words for you to enter. One of them is authenticating your human-ness and the other is translating a word that computers couldn't read when scanning a book. The process is remarkably accurate.
There are now 350,000 sites using reCAPTCHA, which results in people helping translate 100 million words a day, or 2.5 million books a year. The magic happens "one word at a time from people typing CAPTCHAs on the Interent," said von Ahn.
To date, 750 million people (or 10% of world's population) have participated in reCAPTCHA and helped translate at least one word from a digitized book. That's the big "Ah ha!" for Luis and it's what he has chosen to focus his research on, as he explains on his Carnegie Mellon bio:
At the height of its construction, 44,733 people worked on the Panama Canal. The Great Pyramid of Giza required 50,000 workers and the Apollo Project 400,000. No matter what you put on this list, humanity's largest achievements have been accomplished with less than a few hundred thousand workers because it has been impossible to assemble (let alone pay!) more people to work together--until now. With the Internet, we can coordinate the efforts of billions of humans. If 400,000 people put a man on the moon, what can we do with 100 million? My research aims to develop theories and build computer systems that enable massive collaborations between humans and computers for the benefit of humanity. I am working to develop a new area of computer science called human computation, which studies how to harness the combined power of humans and computers to solve problems that would be impossible for either to solve alone.
To me, the crux of this is Luis's question, "If 400,000 people put a man on the moon, what can we do with 100 million?"
Luis is trying to answer that with one of his next projects, Duolingo, which is an attempt to harness the power of millions on the Internet to translate the entire web (or close to it) from English into other languages. There are over 1.2 billion people currently learning a foreign language and language education can be expensive, so Duolingo is a web-based program (coming soon) that will let people learn a foreign language for free and at the same time help translate the web into foreign languages just by participating in the software. You can hear Luis expain more about it at the end of his TED Talk.
I'd highly recommend watching this one. It even includes some good CAPTCHA humor.