Gutenberg: the Audio Books Project
www.gutenberg.org/wiki/Gutenberg:The_Audio_Books_Project
Public domain fiction and nonfiction. Both human-read and computer-generated audiobooks available.

AudioBooksForFree.com
www.audiobooksforfree.com/home
Professional narrators. Free downloads are of "bearable" quality and in multiple MP3 files; options for fewer files and higher sound quality available for a fee. Fiction (mostly adventure and crime), classics, childrens', some nonfiction.

LibriVox
librivox.org
Public domain works of fiction and nonfiction. Download entire book as zip file, download multiple MP3 files, or subscribe via iTunes. Listen on computer, iPod, MP3 player or burn to CD. Narration done by volunteers. High quality sound files.

openculture
www.oculture.com
Alerts users to cultural and educational media freely available on the web, including university courses, foreign language classes, videos, and news.