Love Audible

Don't see this mentioned yet, but personally I think the best deal on Audible is that you can use a single credit to download an entire Great Courses series. It is a pretty steep discount over getting access to them via GC or other apps.

I have used this to download and listen to literally hundreds of hours of lectures on everything from personal finance, economics, art history, philosophy and great works of literature. I highly recommend this for long commutes since most lectures are close to an hour.