- AudioBooksForFree (http://www.audiobooksforfree.com/screen_main.asp)
- Project Gutenberg Audiobooks - includes both human narrated and computer-generated text to speech (http://www.gutenberg.org/audio/)
- Telltale Weekly - some free, some available for purchase at prices ranging from $0.25 to $12 (http://www.telltaleweekly.org)
- LibriVox (http://www.librivox.org/)
Also, if you run across any more sites offering free downloadable audiobooks, let me know, and I'll add them to the list.