When I drive- which is most days - I usually listen to lectures or podcasts downloaded from iTunes. I recently finished an excellent Yale course that deserves a recommendation: Paul H. Freedman's The Early Middle Ages, 284-1000. As the title indicates, the course covers the period from the accession of Diocletian (with a prequel on the Crisis of the Third Century) through the demise of the Carolingian Empire and the arrival of the Vikings. Prof. Freedman clearly loves the age, and his well-constructed lectures guide his students through the difficult material with care and a dose of humor that had me laughing out loud more often than I care to admit.
Earlier recommendations are here.