You can find quite a few free audiobooks online but they're frequently scattered all over. Save yourself the run around by checking out the thousands of free audiobooks cataloged at BookShouldBeFree.
BooksShouldBeFree houses thousands of books in the public domain, available as both MP3s and as iTunes format audiobooks. Each entry for a book includes links to the full text, supplemental text and reviews.
The reviews are particularly helpful because they are specific to the actual audiobook recording and not just the book in general so you get feedback like "The use of character voices in this record was excellent" or "I love the story but this recording was a rather flat reading of the text", information relevant to your enjoyment of the book in audio form.
BooksShouldBeFree is a free service and requires to registration to use. Have a favourite place to find free audio books? Let's hear about it in the comments.