Do you happen to have any credits left on your audible account? Then you most likely got this promotion:
All the two for one deals are nice and since I do happen to have some left over credits, I went ahead and purchased 'Origins of Life' and 'Big History: The Big Bang, Life on Earth, and the Rise of Humanity'.
I hope they turn out to be as enlightening and engaging as their reviews promise to be..oh well we will see!
Moreover, for the month of june, whispersync for voice offers for free The Adventures of Tom Sawyer (I believe you have to purchase the free kindle too to get the free audiobook)
Also there is a promotion for the gold audible membership for as low as 99ct per month! (deal is for two months, then it's going back to normal price of 14.95$) You can enter this promo code: 153IMUXOB324 and you should not be enrolled to a membership program to be eligible for the offer.
I cancelled the plan I had and enrolled to this one as it's sooo cheap!