The first place in the iTunes U Top Download list is a lecture by Hubert Dreyfus, professor of philosophy at UC Berkeley. The lecture, "Introduction: What is Existentialism?" is from his Spring 2006 course on existentialism in literature and film. I wondered how philosophy could take first place in such a list, but the description mentions religious issues which might explain it.
The NYTimes calls Walter Lewin a web star and says he "lectures with the panache of Julia Child bringing French cooking to amateurs and the zany theatricality of YouTubes greatest hits." He's a 71 year old physics professor from M.I.T. But my listing at the iTunes shows more lectures from Leonard Susskind's physics class at Stanford in the top 20.
Michael Dell's talk at Duke is number 20 at iTunes U, but Steve Jobs commencement address at Stanford is number 3.
iTunes U is a great service, but most of the content is also available elsewhere if you happen not to use iTunes. The Dreyfus lecture is available for download from UC Berkeley. Lewin's physics course is part of MIT's OpenCourseWare program. This has been improving lately so if you haven't checked it out in the last year or two, try again.
Related: Ian Lamont: MIT's OpenCourseWare project sets an example for higher education