I've gotten hooked on the Portable Professor Series of audio CDs. Each set is a series of college lectures on a single subject, and the choice of subjects ranges from the American Revolution to the Bible. I've also tried listening to audio books in the car, but these lectures seem more engaging, because they were designed for listening, rather than being written content read aloud. Each lecture lasts about 30 minutes, so I can listen to one lecture per day's commute. I'm currently listening to the series about the American Presidency in the 20th century, but my favorite so far was the series on ancient Rome.