Free Classic AudioBooks has a nice list of audio books available online for free. They are all read by text-to-speech software, but the result is by far the best I've heard.

I'm listening to Swiss Family Robinson while I work, for free! Go straight to the first mp3 file for a quick sample.

Here is the full list of books from their front page, put into alphabetical order.

• Aesop's Fables

• Aladdin and the Lamp

• Autobiography of Frederick Douglas

• Buffalo Bill by W.F. Cody

• Cattle Brands - Western Short Stories

• Humorous American Short Stories

• Imitation of Christ by Thomas Kempis

• Jekyll and Hyde by Robert Louis Stevenson

• Legend of Sleepy Hollow by W. Irving

• Orthodoxy by G. K. Chesterton

• Pride and Prejudice by Jane Austin

• Robinson Crusoe by Daniel Defoe

• Russian Short Stories

• Swiss Family Robinson by Johann Wyss

• The 911 Commission Report

• The Call of the Wild by Jack London

• The Devil by Leo Tolstoy

• The Scarlet Letter by Nathaniel Hawthorne

• War of the Worlds by H.G. Wells

Update!

There is an online project, Librivox, that does the same thing (free, public domain audio books), but with real humans reading the books. See their (sadly, brief) catalog.