There is always the Audible app. Most of the audio books aren't free, but they have a large selection.
Audiobooks.com also has an android app. I haven't used their service, but they have a pretty good selection.
There is also podiobooks.com, which has a lot of free indie audio books. They don't have an android app, although books/chapters can be delivered as a podcast (you can use a podcast app to listen to books). I've found some interesting selections there.