These are the best sources of free audiobooks I've found. Most of their books are in the public domain, and many are classics. For more recent books, check with your local library; many let you download audiobooks, even from home. My local library, Herrick District Library, provides audiobooks from several sources.
Where do you get audiobooks? Tell us in the comments!
If you like audiobooks, you may also like podcasts. Read my post Listen and learn with podcasts.