I've gotten into audible.com lately.
There's a ton of stuff you can download onto your computer and transfer to mp3's, discs, etc. It does cost, but... there are always cheapie sales. And if you become a member, you get a pretty good deal with getting to choose a number of titles each month, etc, that turns out being much cheaper than if you bought them.
I was just renting audio books from different places, but audible has a much better selection. I've been finding some really cool things.