This is my most recent discovery, and I'm already hooked.
"LibriVox volunteers record chapters of books in the public domain and release the audio files back onto the net. Our goal is to make all public domain books available as free audio books."
Anything that was published before 1923 is now free of its copyright, and so this volunteer-run site, LibriVox, is endeavoring to create audiobooks of those works. Way cool.