ManyBooks.net is a website that aggregates free eBooks and audiobooks for your PDA, iPod, or eBook reader.
The site makes it easy to sort through ebook titles, categories, and authors. There is also a reference section with a variety of encyclopedias, dictionaries, and other useful books for download.
Everything is absolutely free – including their list of audiobooks.
You can also subscribe to their RSS feeds to get an updated list of free eBooks or Audiobooks sent directly to you.