Amazon launched it's Kindle Unlimited service ($10 per month, but you get a 30-day free trial to start. You can have up to 10 books at a time.). I still have more to go through, but I've been looking for things where if you read for free via the Kindle Unlimited free trial, you also get the audiobook for free or cheap. It looks like if you return the Kindle format book or cancel your service, it will also delete the audiobooks that were free from your account (though you could download to iTunes or somewhere else to keep them probably...UPDATE: it does not appear this is the case. The free ones are not available to download.). I'm guessing you get to keep the ones you buy (UPDATE: This is correct.), even though you purchased them at a discounted price. Here are a few of note:
Free audiobooks with free download of the Kindle book:
Happy Accidents: Serendipity in Major Medical Breakthroughs in the Twentieth Century by Morton Meyers MD (a Nassim Taleb book recommendation)
Life on the Mississippi by Mark Twain
The Adventures of Huckleberry Finn Tom Sawyer's Comrade by Mark Twain
The Man Who Mistook His Wife For A Hat: And Other Clinical Tales by Oliver Sacks
The Autobiography of Benjamin Franklin
Capital in the Twenty-First Century by Thomas Piketty (I was reluctant to list this given that there seems to be errors and some data mining in this remarkably popular book, but in case you were considering reading and/or listening to it, free isn't a bad price to pay for it.)
The God Delusion by Richard Dawkins
The Rough Riders by Theodore Roosevelt
Discounted audiobooks with free download of the Kindle book:
How Children Succeed: Grit, Curiosity, and the Hidden Power of Character by Paul Tough ($3.99)
Mistakes Were Made (But Not by Me): Why We Justify Foolish Beliefs, Bad Decisions, and Hurtful Acts by Caroll Tavris and Elliot Aronson ($3.99)
The New New Thing: A Silicon Valley Story by Michael Lewis ($4.49)
Home Game: An Accidental Guide to Fatherhood by Michael Lewis($3.99)
Onward: How Starbucks Fought for Its Life without Losing Its Soul by Howard Schultz and Joanne Gordon ($4.99)
The Theory of Relativity: and Other Essays by Albert Einstein ($3.49)
The World As I See It by Albert Einstein ($2.99)
Essays in Science by Albert Einstein ($3.99)