Free audio books are available from organizations such as LibriVox, Lit2Go and Internet Archive. Books at these organizations are in the public domain and narrated by volunteers. Audible, an Amazon company, provides a free audio book if you sign up for a paid membership. These include bestsellers and new arrivals.
Some authors release complimentary audio versions of their books on their personal websites or iTunes. On iTunes, listeners subscribe to the podcast to receive a chapter as it is released. Podiobook.com is another website that offers free audio books from independent authors. The books are free but donations are accepted.