According to Apple, one of the most popular universities on iTunes U has been Open University (iTunes link), the UK-based educational organization dedicated to distance learning whose programs include Arts and Humanities, Business and Management, Childhood and Youth, Health and Social Care, Law, Psychology, and Science. OU says they have around 150,000 undergraduate and more than 30,000 postgraduate students.
In related news, if you've never watched Steve Jobs' Stanford University commencement speech give it a download [iTunes link]. It's emotional, it's raw, and it's when he revealed to the world he had cancer. You can also read the text of his speech here.