Of course Audible is owned by Amazon, but this is a new feature I just found out about. They are now starting to release versions of audiobooks that support Whispersync. So going back and forth between the audiobook and ebook, they keep track of each other.
They also discount the price of the audiobook if you aren't an audible subscriber. I looked at the newest Daniel Suarez book, the audiobook went from $23 to $10.
Though there are some books where it would seem like losing something by going to the ebook version. Ones like Stephen King's Dark Tower books 2-4 come to mind as big examples.