I listen to a lot of Teaching Company lectures. You can't get them on Audible/iTunes, though. You can get some good stuff via iTunes U though.
Seamus Heaney reading his Beowulf is pretty great. I also listen to a lot of PG Wodehouse in audiobook, both readings of the novels and dramatizations.
But for the most part, lightweight nonfiction works best. For example, I recently enjoyed Empire of Blue Water, Liar's Poker, abd The Devil in the White City as audiobooks.
My last road trip involved catching up on a Comedy Death Ray Radio backlog.
Protip: if you use iTunes, make sure that things are actually set to a media type of audiobook, with bookmarking enabled. Then on an iPod they show up in the right place, are bookmarked, and you can listen to things at 120% speed, pitch-shifted.