Just recently – I found out yesterday – the University of California, Berkeley, made available a whole swathe of lectures available through the iTunes Music Store. Their website on this is here, which has the gateway link into the relevant part of the store.
It’s worth noting that Apple are providing the hosting for all this. They’re hosting a colossal number of lectures, each very large. The bandwidth consumed must be incredible. Kudos to them.
Yes, you’ll need iTunes to access them.