I also love audiobooks! We use a lot of audiobooks in our schoolwork. We have listened to several books from our curriculum from mp3 files on the computer or CDs in the car. Currently we are listening to An Island Story and Burgess Animal Book for Children on Librivox. I have become a big fan of Librivox, an online provider of free audiobooks from the public domain. As a general rule, if it's a classic book, it's on Librivox.
Since I'm frugal to the core (I prefer the term "smart"), I also utilize my library for a lot of audiobooks on CD. Right now, we're listening to Farmer Boy on CD. We've listened to all the Little House books this way. We listened to Pinocchio, Alice's Adventures in Wonderland, and Wind in the Willows while traveling. Listening to books in the car eliminates sibling bickering, and since they have nothing better to do, they love it!
Another gem I was recently introduced to is Old Time Radio Programs. "You Are There" is a fantastic program for teaching kids history. You can listen to a news reporter reporting events of history as they happen. Try the episode The Sailing of the Mayflower. After you try one you'll want to use the rest to supplement your history studies.