Just enter "audio books for free" on Google and a whole load of sites come up.
I don't think many of them are entirely free, but you'll be able to browse what's on offer.
Good listening...
Project Gutenburg have a small selection of audiobooks. Like the rest of their offerings, all the titles are out of copyright. If you want the latest bestsellers then you will not be able to get free legal downloads (unless it's some sort of enticement to get you to sign up to a service).