Not tried them (so I don't know about the quality), but if you want classics, Project Gutenburg (they archive out of copyright titles) have some audio books. The human-read selection (
http://www.gutenberg.org/browse/categories/1 ) only has "A Christmas Carol", but the computer-read selection (
http://www.gutenberg.org/browse/categories/2 ) seems quite extensive.
mad4music's Limewire comment is correct but (as with most P2P content) please bear in mind that while free, what you are downloading is not legally traded.