If you have a kindle amazon is the best place for you, if not http://www.booksshouldbefree.com/ is pretty decent. http://www.newfiction.com/ [account required] and http://www.openculture.com/freeaudiobooks all are ok, just if you want a wide selection your going to have to pay. depending on your personality and book preference its all up to you if they have what you want.
You can find some more free audio books here. http://www.pickthebrain.com/blog/free-audio-book-and-podcast-resources/
