The free trial you are possibly referring to was www.audible.com/sansa08. It’s expired, but the link is still active as of today. I didn’t try this offer.
I’ll second librivox.org for the classics.
You may be able to find a local library participating with OverDrive.com - lots of titles available to download. I haven’t tried this serivce yet since I don’t have a reliable internet connection.
