There are several sources for audio books. I will start with the free providers. The next blog post will cover other sources.
Guttenberg Books in public domain; read by volunteers or computers
LibriVox Books in public domain; read by volunteers
Your local library is the ultimate free source of audio books. Check with your local librarian for the rules on burning copies of CD books to your personal devices, like iPods or mp3 players. If you library has the Overdrive system, you can download the books directly to your device and listen them for whatever the lending period is. The York County Library system has a one or two week lending period that the user chooses when setting up their Overdrive account.
There are several "free" apps on iTunes and other app stores for audio books. Look at them carefully before installing. Many of them come with one or two books from librivox or gutenberg and then charge for additional books. Most of what they are charging for is already free.
Checking a book out from the library is a great way to make sure you like the narrator and style before you buy the audio book.