MoMA (the Museum of Modern Art) in New York City has recently released its lectures series for free on iTunes U. You can now listen to world-class artists, critics, and scholars on a range of topics: from ancient art to abstract expressionism.
The lecture series are in audio formats as well as video series lectures. These are beautifully presented on both your computer screen or iPod. You can view their lectures series availabilities on iTunes U via this link.