Mp3Raffel includes a large collection of public domain books, most of which are audiobooks that have been downloaded from the National Digital Library Program. While it leans a bit more on fiction, many of the audiobooks are actually quite entertaining and educational.
The Free Book Index features ebooks available from a variety of publishers, including many smaller publishers. A good thing about this collection is that it tends to be more high-quality than most of the free ebook sites around, so even if you don't find audio playback on this site, you can read the book's PDF and view the epub of the added ebooks.
The National Public Radio has committed to dozens of years of archives, much of it captured by the Internet Archive. While many of the ebooks are fiction, you can also find plenty of audiobooks here, for issues dating back as far as 1892. The collection gets you started with a search page, but it's not easy to get logged-in to NPR.
A search engine for public domain books that promote environmental issues. How To Clean Your Windows has 20,000 public domain books and audiobooks on its site and has also developed a really cool Chrome extension that provides easy access to its ebooks.
The University of Chicago Press has the largest collection of public domain audiobooks on the web. It's still a relatively new offering and is currently limited in length to two hours. Nonetheless, the program does offer an easy way to find materials in the public domain. And it is a lot like iTunes for audiobooks, in which you can quickly play or download through your web browser. d2c66b5586