It's not a true freebie - you have to sign up for a service but you can cancel it at any time and keep the book.
Copy/Paste from theirs page:
"Download a FREE audiobook when you try Audible.co.uk FREE for 30 Days
*Try our '1 book monthly' membership for 30 days and choose any one of our 60,000 titles for free. You will not be charged during your free trial period and even if you decide to cancel you can still keep your free download. After your 30 day trial, your membership will renew each month for just £7.99 per month so you can continue to receive one credit per month to use on any title of your choice. No commitment, you can cancel your membership at any time. Any unused credits will be lost at cancellation. Free trial offer only available to first time members of Audible, if you are not currently a member and have not had a free trial or Audible membership before. You must be over 18 to use the Audible site. Credit card details required to verify age and identity but your card will not be charged during your free trial."
Standard subscription offer (same for Downpour) - not a special deal.