iTunes U from Apple offers courses from a large variety of colleges and universities as well as some primary schools. iTunes U is accessible directly through iTunes on Mac or PC as well as through an app on IOS 5+ (iPod and iPad) devices.
The courses featured range from excellent to painful. We have found that the major institutions tend to have better materials and lectures but would recommend going through a lecture before downloading an entire course to see if you like the teaching style and overall presentation.
iTunes is available free from Apple at: http://www.apple.com/itunes/