Over the past week or so, I've been listening to a series of lectures from Ohio State University Astronomy Department .  I use iTunes on my Mac laptop to pick the lectures up from the 'iTunes U' section of the 'iTunes Store' as podcasts and then listen either on the Mac or on an iPod.  So far I'm around half way through Astronomy 161 given by Prof. Richard Pogge and find it very interesting although the American pronunciation can take a while to get used to.

(update: I've managed to catch up with the podcasts and eagerly await each new one.)