[olifante]

I have to second that recommendation. I've been studying Pimsleur's excellent Mandarin Chinese I, II and III in my daily commute and road trips for some time, and I highly recommend it. It requires reasonably active listening and sentence production, so it helps you avoid drowsiness on long trips.

Coincidentally, I'm leaving soon on a car trip to Morocco and was just preparing to sync the free Michigan Modern Standard Arabic and the Pimsleur Egyptian Arabic audio courses to my iPod.