I used to subscribe to Audible.com, and really enjoyed it. I could get a couple audio books every month and really enjoyed it. But it was a little spendy. At $15 per month, it adds up really fast. So I canceled my subscription.
I still enjoy audio books but I tend to get them from other places. For example, libraries often have a good selection of books on CD. Also, you can purchase individual books from a few sources.
Now, I have found that more companies are offering audio books online. I came across LearnOutLoud.com, and it looks awesome. LifeOptimizer.com has rated them in the top five best learning tools on the internet (They placed third).
They have a combination of free and for fee audio books. They also include some bonus features, like giving customers the opportunity to either download or purchase a CD.
To top it off, they host podcasts and allow publishers to sell their audio productions online. Then they give you a 60% royalty on the sales.
Keep an eye on this company, they will be a major player in this market in the year to come!