For those who don't know, the founder is Luis von Ahn[0], a CMU CS professor. I know someone majoring in computer engineering there who's taken one of his courses, and apparently he's a very engaging instructor as well.
I saw him present his talk about "harnessing the power of people doing meaningless things on the internet" at an AI conference back in 2003, where he introduced what became the Google Image Labeler.
Easily the best talk of the conference - he's a great (and engaging) speaker.
I think this google talk is roughly the same presentation:
I've done research with Luis, and some of my friends are working on Duolingo. He's certainly an interesting person and a great lecturer. You really come to appreciate his teaching skills after taking his course (most take it as freshmen) and realize all the other professors are nowhere near that competent in teaching.
dissenting opinion: It's easy to teach an interesting course when you get to handpick all of the material. 251 is called "Great Theoretical ideas of Computer Science" because it covers many of them. Alternatively, it's hard to come in and blow your students away with matrix algebra day after day.
His other popular class is called "Science of the Web," which is similarly open-ended.
He's undoubtedly a great teacher, but the other professors are also very competent; albeit in less sexy topics.
small counterpoint to your counterpoint: I had him for 15-381 (Intro AI for non-CMU people), it was his first time teaching the class, and his lectures were substantially better than those of the other professor he co-taught with and had been teaching the class for many semesters. He was either my favorite or second favorite CS prof at CMU.
I worked with him in my undergrad research. Luis is one of those people who communicates with such ease and simplicity that not only is it obvious he's extremely smart, he actually makes you feel smarter as well.
0: http://www.cs.cmu.edu/~biglou/